Hold Command-Option-P-R on startup and keep them held until you hear the second bong startup noise.
If it doesn't boot after that, try booting in safe mode.
Hold the left shift key on bootup until after you see the spinning wheel on the white apple screen.
If that works, repair your disk permissions* while booted in safe mode then restart the computer and you should be fine.
* to repair disk permissions open applications/utilities/disk utility select the main hard drive and under the First Aid tab, click the repair disc permissions button.