Originally Posted by dustijane
Always backup before running a software update. You do back up don't you? Hmmm, I guess not since you're worried about losing stuff. So I guess you don't really
care that much about this stuff, or you would have backed it up. Hard drives can and do fail with no warning, and this affects Macs just as much as PCs. Buy an external hard drive and back up to it on a regular basis. OS X has Time Machine built in, but I use Carbon Copy Cloner which can create a fully bootable copy of your internal HDD.
In terms of your more immediate problem, try removing the mains power and battery, then pressing and holding the power button for several seconds. Then, replace the battery, plug in and try turning on again. If this doesn't work, book a genius bar appointment (they're free).