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.
The prohibitory sign means there are missing files from your System Folder that are required to start the machine. Do you think it is possible that you accidentally deleted some necessary files from the system? This can happen when someone deletes a program manually by trashing all of the files that have the name of the program in it.
If you didn't delete anything, try the following steps:
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. 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...
FSCK REPAIR IN SINGLE USER
Next, let's try to run repair from the command line 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.
1) Start the mac and hold down the apple key (Command) and the S key until the machine starts into Single User Mode.
2) 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 this command (in bold) EXACTLY(there is only a space between "k" and "-") and then press return:/sbin/fsck -fy
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.
3) When fsck reports that your volume is OK, type the following (in bold) EXACTLY and press return:reboot
Does the machine boot up now? Or are you still stuck? If this corrected the problem, STOP HERE. If not, go on to the next step...
ARCHIVE AND INSTALL:
The next step would be to "Archive and Install" the Mac OS X. The "Archive and Install" option on 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