Try starting up in single user mode and repairing the disk:
1. Reboot and hold command+s
2. When you get the prompt, type fsck -yf
3. When complete, it will either say DISK IS OK or FILESYSTEM MODIFIED - if you get the second message, run fsck -yf until you get the first message
4. When done, type exit
If that doesn't work, you could try booting into Safe Mode by holding the shift key at startup. And if neither get you going, you'll need to reinstall OS X.