The usual cause is that the hard drive has gone bad... But you may just need to re-order your BIOS?
Have your tried borrowing another drive just to see if it is detected?
That computer can be about 2 1/2 years old... long enough for a drive to go bad...
But if you have valuable data on it, you don't want to mess with it too much... just because it doesn't boot, or is not detected, doesn't mean you cannot access it later to retrieve the data... because usually you can.
It can be a lot of other problems as well, so tell us more about your system and what you were in the middle of?