Dell E 510

Usually this means the disk drive has failed. Open case. power up. See if you can hear the hard drive spinning.

If the hard drive failed, no big deal except you've lost your data. Get a new disk drive. The use the recovery CDs you burned to restore your PC to the factory load. If you lost / didn't make the recovery CDs call Dell. They will cost you $10-$20 to get.

Aside: if losing your data was a bummer then start backing up your disk to an external USB drive. Win7 comes with it's own backup software.