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 its 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?
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 at least 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 an Firewire cable to connect the two, I would try this first.
1) Shut down your troubled Mac.
2) Start up the good Mac.
3) Connect a Firewire cable between the 2 machines.
4) 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.
5) This should connect the troubled Mac to the good Mac as an external hard drive.
6) You should then be able to backup the troubled Mac to another external drive or to the good Mac via Carbon Copy Cloner.
If you don't have access to another Mac or the Transfer mode startup didn't work, then try the following:
FORCE STARTUP FROM YOUR SYSTEM FOLDER:
Hold the "X" key during startup immediately after pressing the power button (Don't let go 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. 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.
BOOT FROM THE STARTUP MANAGER
1) Restart while holding down the Option key immediately after pressing the power button (Don't let go 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. It 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.
1) 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.
2) 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 sounds (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). Does the machine boot up or get stuck again?
3) 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 until it starts from the disk or locks up). If you have to choose, select the Installation disk as the startup. When it starts, cancel the Installation process. Then go to the Go menu, down to Utilities. Choose the "Disk Utility" program. Run Disk Utilities. 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?
4) 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. Release the keys. This resets the PRAM on the machine.
5) 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, and then click the Repair Permissions button. This may take a while. When finished, quit Disk Utility.
6) Restart normally.