Q: computer won't boot past gray screen

If you hold down the Shift key when restarting it will put you into Safe boot mode which is the gray screen. Make sure your Shift key is not stuck. Remove your keyboard if possible then restart.

Try this option to reset PRAM

Press Option-Command-P-R until you hear startup sound a second time.

If you have your Snow Leopard CD, insert the Cd and restart holding down the C during startup.

http://support.apple.com/kb/ht1533

Try running Disk Utility > Repair Drive

Startup - Gray, Blue or White screen at boot, w/spinner/progress bar

Startup - Gray Screen

Startup Issues - Resolve

Startup Issues - Resolve (2)