Pray that your hard drive is okay..
Usually this means your computer can't see the hard drive which usually means you just lost your...HD
Shut down the computer.
Locate the following keys on the keyboard: Command, Option, P, and R. You will need to hold these keys down simultaneously in step 4.
Turn on the computer.
Press and hold the Command-Option-P-R keys. You must press this key combination before the gray screen appears.
Hold the keys down until the computer restarts and you hear the startup sound for the second time.
Release the keys.
Your computer's PRAM and the NVRAM are reset to the default values. The clock settings may be reset to a default date on some models.
It has worked for me in the past once with the same issue.
It also didn't work for me once
X423424, while your help is always appreciated, I would like you to refer to my post and read that it has fixed my problem and that was over a year ago, no problems since. In fact Apple would tell him to do the same thing as a first go. So don't tell him it won't work. it may or may not. I am living proof of may.