How do you fix a computer that keeps saying insert boot disk

One of two things is happening:
1. The system cannot find the hard drive with the OS on it.
Check the BIOS to make sure that it points to the boot drive/partition (generally the C: drive).

2. The OS has become corrupted.
You can try to do a repair, but you may need to do a complete reinstall.

Could also be that the C: drive died, which is why the BIOS cant find it.

Double check power and data cords to the hard drive.

if BIOS doesn't see a hard drive installed, (if you cannot select the hard drive to boot from) the circuit board for the HDD is fried. If you have more than one hard drive, make sure you can select the same number as you have installed.
If Bios sees the HDD but still asks for boot, the drive was likely corrupted and can usually be fixed by OS repair.

Make sure that if you have multiple HDs, your BIOS is set to attempt boot from the drive with your OS.