computer will not boot up
When I turn on the computer (iMac), I see the Apple sign and the timer circling and nothing happens. The computer is not booting up.
iMac
See this Apple note on those non-boot symptoms. If you are running OS X 10.7 or 10.8, when the note refers to booting from DVD, boot into recovery holding the command and r keys down, in order to do a Disk Utility disk repair.