Why is my Compaq presario saying operatig system is missing

There is a good chance that something negative has happened to your hard drive.

I would suggest going into your bios first. (When you first boot the system, hit the button that will give you access, normally ESC, DEL, or an F key)

See if you can locate your hard drive (it may be referred to as HHD).

If it lists the HHD, then you are probably experiencing software issues.

If it is not listed, it is more likely a hardware issue.

I can post additional help after you get to this point, in the comments.