|The bios can't find the system files on the disk it's trying to boot from Sometimes that's as simple as removing a floppy disk that was left in the drive.|
Or the hard drive data cable may be damaged or loose. Or the hard drive may be bad.
Go into cmos/bios setup and make sure the drive is correctly seen there.
Also, what happened to the computer between the last time it worked OK and the first time it didn't?