Do you have a floppy in the drive (if you have a floppy drive that is), CD in the CD tray or a USB flash drive plugged in? Remove any media from the computer and reboot see if it comes up.

Go into the BIOS, see if the hard drive shows there, your drive may have failed.
 
This is caused by the system not seeing the boot device that it's supposed to load from. Either the boot order was changed, the hard drive is malfunctioning, or the hard drive got disconnected.

Tap the delete key repeatedly on startup. That should get you into the BIOS. Make sure the hard drive is the first boot device. If it isn't, change it so it is.

If that doesn't resolve the issue, remove the side of the tower, and make sure the cables to the hard drive are connected securely.