You can try booting into single user mode and running fsck from the command line. To do this, hold down apple and s as the machine boots. You should get a command prompt. At the prompt, type "/sbin/fsck -fy" without the quotes. Make a nice cup of tea to drink while you wait. If it repairs anything, you'll see it tell you so. It should eventually end with something like "Macintosh HD volume OK" (or "Volume Macintosh HD OK"). If it ends with anything else, run the command again.
Type "reboot" to restart the computer normally. You should be sure to repair permissions using Disk Utility. Either boot from the CD/DVD to do this (if you can) or do it once logged in (if you can).
You can also try a "safe" boot. Hold down shift while booting. This may let you boot to troubleshoot.
If this doesn't help, I'd suggest reposting with as much detail as you can - version of OS X, what happens when you run fsck / safe boot etc. That will help people reading narrow the problem down and maybe make some more specific suggestions. What I've suggested here is generic advice - it certainly cannot do any harm, but it may be that you'll need to post more information if the issue is more specific.
Good luck,
cfr
