There is a few things you can do,
BUT NEVER JUST HIT THE POWER BUTTON!
One open a Terminal if you don't know where one is, try this hit CTRL+ALT+F1 then logon with root if you can but you'r user name should work, any was after you loged in type halt or reboot. Halt is to shutdown, and well I think you should know what reboot does
-T0ny