If it a very old computer, a floppy disk forgetten in the drive can cause that. Removing the disk will fix it. If it is an old computer, the HDD may have died and no disk with a boot sector is available. A new HDD will fix it. If it is a recent computer, maybe the boot order get messed if you tried to start the computer with an usb memory stick or SDcard in an usb reader. Happen often on Asus board. Removing the card or the usb stick, and setting the proper boot device in BIOS will fix that.
Also, If you are comfortable with removing the side panel I would make sure the power plug - white with 4 wires going into it - is pushed all the way in. I have and old one that the molex, white power plug seems to slip out and I get the exact same error you got.