Can you do a safe start by holding the shift key down when booting? This will take longer than normal. If that works, try a normal boot.
If that doesn't work, find an OS X DVD, insert and boot to it by holding the C key down (or the alt/option key and choose the OS X DVD), then bring up Disk Utility to repair the hard drive, and also repair permissions while there. Then try to boot normally, assuming the disk repair completes without error.