If the following suggestions work, please let me know and you can Accept the answer (and if possible, leave positive feedback). If not, please DO NOT Accept the answer. Instead, let me know and we'll try some more things or I can refer you to another Expert who may be able to help you further.
Before we go any further, I would suggest you start using a program like Carbon Copy Cloner (http://www.bombich.com/
) to create a restorable backup of your Hard Drive to an External Hard Drive. This free program will make a mirrored copy of your hard drive in it's current state that you can go back to if things go south for any reason. I do this personally before I perform any kind of install, update or major change to my system. Then if you find yourself in this type of situation, you aren't as frantic. BUT, you don't seem to have a backup right now, so let's get your machine up and running, shall we?
1) Firewire Transfer to backup what you can
Do you have access to another mac? If so, now would be a good time to try to connect the two machines in Firewire transfer mode, that way you can atleast get your important stuff off of the machine if it is able to boot up into Target Mode. If you do have another mac close and a firewire cable to connect the two, I would try this first. If you don't have access to another mac or it won't start into transfer mode, skip down to step 2 "Force Startup from your System Folder"
1a) Shut down your troubled mac.
1b) Start up the good mac.
1c) Connect a firewire cable between the 2 machines.
1d) Press and hold the T button and then power up the troubled mac. Don't let go of the T until the firewire logo starts bouncing around on the screen.
1e) This should connect the troubled mac to the good mac as an external hard drive.
1f) You should then be able to backup the troubled mac to another external drive or to the good mac over the Firewire Connection
If you don't have access to another mac or the Transfer mode startup didn't work, then try the following:
2) FORCE STARTUP FROM YOUR SYSTEM FOLDER:
Hold the "X" key during startup immediately after pressing the power button (Don't let go of the "X" until it starts from the disk or locks up). This may force the computer to start up to Mac OS X. If it does, open System Preferences and be sure that your Mac OS X System Folder is selected as the startup disk. If this works, restart after you have your startup volume selected. If this corrected the problem, STOP HERE. If not, go on to the next step...
3) BOOT FROM THE STARTUP MANAGER
Restart while holding down the Option key immediately after pressing the power button (Don't let go of the "Option" key until it starts from the disk or locks up). This should bring up the Startup Manager and would allow you to select the startup volume. May work, if the computer is stuck and can't find the correct startup volume. If this works, open System Preferences and be sure that your Mac OS X System Folder is selected. Restart after you have your startup volume selected. If this corrected the problem, STOP HERE. If not, go on to the next step...
Follow the steps below and let me know what each one yields. I'll check back all day and see if we can resolve this issue.
4) Locate a Hardware Conflict
Unplug everything (ipod, usb hard drive, printer, etc) except the mouse and keyboard from the computer. Then restart. Does the machine boot up or get stuck again? If it starts up, plug each thing back in one at a time, restarting the machine each time until it locks up. Remove this item and continue adding items until you have everything plugged back in that you can and still have the machine restart successfully. Let me know which peripheral(s) locks the machine up. Regardless of whether or not the mac started up, move on to step 5.
5) Safe Boot Startup
Plug everything back in (but make a note of what caused problems for later troubleshooting). Start up the computer in safe boot mode. To do this, press the power button. As soon as you hear the startup sound (immediately after you hear the sound, but not before), press and hold the Shift button. Let go of the Shift key as soon as you see the gray apple icon and the progress bar (or wheel). This startup method may take 20 - 30 minutes, don't be alarmed. Does the machine boot up or get stuck again?
6) Disk Repair
Insert the Mac OS X install disk. Start the machine while holding the C key immediately after pressing the power button (Don't let go of the "C" until it starts from the disk or locks up). If you have to choose, select the Installation disk as the startup. NOTE: If you can't get the mac to startup from the Install Disk, try starting up from the Install Disk in an external DVD Drive.
(it sounds like you are trying to go this route already). Try the disk in the external drive and booting from the Startup Manager (Step 3 above). You'll know you are running from the Install Disk, when it starts up to a screen asking you to select your language. If you can't get it to start from the Install Disk at all, skip down to step 11, "fsck Repair in Single User".
When it does start from the Install Disk, choose your language and then don't go any further. Instead, go to the Go menu, down to Utilities. Choose the "Disk Utility" program. Run Disk Utility. Depending on which version of Mac OS X Install Disk you have, this may just be a "Utilities" menu, then "Disk Utility".
Select your hard drive in the column on the left and "Verify" the Hard Drive. Repair any errors that it may find by next choosing the "Repair" button. When finished, restart the computer normally. Does the machine boot up or get stuck again?
7) PRAM Reset
While the computer is shut down, locate these keys: command (has an apple on it), option (sometimes called alt), P, and R. Turn the computer on, then press and hold Command-Option-P-R before the gray screen appears. Hold these keys down until you hear the startup sound a total of 3 times (this can take a couple of seconds between chimes, or a couple of minutes). Release the keys. This resets the PRAM on the machine.
8) Repair Permissions
Startup the computer normally (not off of the Install CD) with everything plugged in. When (if) the machine starts up, go to Applications / Utilities / and run "Disk Utility". Choose the Hard Drive on the left, then click the Repair Permissions button. This may take a while. When finished, quit Disk Utility. If the mac won't start up still, then skip down to step 10, "Archive and Install".
9) Restart normally.
10) Archive and Install Mac OS X
The "Archive and Install" option when reinstalling the Mac OS X will preserve your user data and files, but you may need to reinstall third party software and fonts. Because of this I highly suggest you get a good backup of your hard drive prior to doing this if you can. I would use Carbon Copy Cloner (mentioned earlier).
There is a comprehensive website detailing the Archive and Install process online. I recommend you read through it thoroughly before proceeding.http://www.thexlab.com/faqs/archiveinstall.html
GOOD LUCK! If you get stuck or the mac won't startup at all from any of the above, there are a couple more things you can consider:
11) fsck Repair in Single User mode
This will include commands that will need to be typed exactly as I have them. I'll put them in bold to make things easier.
11a) Start the mac and hold down the apple key (Command) and the S key until the machine starts into Single User Mode.
11b) After the Apple logo (if the hard drive can mount), it will have a black screen with white text. You can release the apple key and the S key. When it finally stops loading and you are given a prompt, type the command below (it's in bold so you know what exactly to type) EXACTLY as I have it (there is only a space between "k" and "-") and then press return:/sbin/fsck -fy
11c) fsck will go through five "phases" and then display info about your hard disk's state. Once it finishes, it'll display this message if no issue were found:
** The volume (name_of_volume) appears to be OK
If fsck found issues and has done anything, it will display this message:
***** FILE SYSTEM WAS MODIFIED *****
If the above message appears, repeat the fsck command UNTIL fsck tells you that your volume "appears to be OK"
The first-pass repairs may uncover additional issues, so it's normal to have to do it more than once.
11d) When fsck reports that your volume is OK, type the command below (it's in bold so you know what exactly to type) EXACTLY as I have it and then press return:reboot
11e) Does the machine boot up now? Or are you still stuck? If not, STOP HERE. If you are stuck, try the last and final thing I can suggest...
12) Remove / Replace Memory
Last thing is try removing one of the memory sticks in the machine and starting up if all of the above has failed. If you still get a freeze or panic, then put this one back in and remove the other one, etc. It may be bad memory. What we are trying to do is figure out what exactly is causing the problem. Then we'll know how to combat it.