First check your BIOS setup. See what it says on the first screen about your hard disk. Could be you need a new internal battery.
When the battery dies the BIOS loses all the info regarding the hard disk and the system can't boot from it.
Turn off your pc.Unplug it.Open the side and unplug/replug the hard drive power cable. no need to touch anything else and it might be as stupid as that. Restart the computer. (i have this problem on mine every now and again, and I know it happens to other computers as well....
good luck.