Boot to single-user mode: restart, while holding Command and S. You'll get a screen with text scrolling down.
At the prompt, type fsck -fy
then press return. That will run a quick hard drive directory check. If that finds any errors, run the same test again, until no problems are found.
Then, type mount -uw /
and press enter again. Be careful that you have a space between the -uw and the last /
Then type exit, and press return again. Your Mac should then restart, so hold your shift key until you see both the grey apple, and the rotating gear under that apple. You can then release the shift key. You'll boot to Safe Boot mode, which prevents certain startup software, and you will get a login screen for that boot, even if your Mac normally logs in automatically.
Does your Mac then go to a normal desktop? Then, try a normal restart.
This process may get you going again.
If not, you'll need your installer CDs, or another set that will allow you to reinstall OS X. Come back here, and someone will help you decide what you need to get.