Macbook Pro Not Booting After Update

Hey guys,
I installed the latest OS X Lion update to get all of the iCloud features but now my Macbook Pro 2007 won't even boot anymore! Whenever I click the power button, the apple logo flashes, but then a grey loading bar appears underneath. After the bar gets about 1/5 of the way there, the computer just shuts off. I have tried resetting the PRAM and disconnecting everything but the power cord, but it has yet to fix the problem. Does anybody else have this issue? Can anybody help me? I need my mac and it has important documents on it.
Thanks in advance

Try rebooting to the Recovery partition and run Disk Utility. Hold down the Command + R key as soon as you hear the chime. After it boots, select Disk Utility, highlight the hard drive, run verify and repair.

Didn't work. Thanks though. Going to use time machine!