First thing to check is look to see if anyone has left a cd rom or dvd in the disk drive, some systems on start up look for a bootable drive which may include looking in that location. So make sure none present.
Then reboot, if problem still exists then it could be a number of things;
1. check cmos settings (usually something like F1 for set up when you power up or F10, depends upon what make of computer. Check hard disk is recognised and is set to boot from hard disk
if not recognised
2. check wiring connections to hard disk and check connections from hard disk to power supply and mother board, could be a loose cable
3. If still not working then could be a hard disk failure