Have you ever switched on you system and you see a message like this:Os not found?
Posible causes:
1.corrupt or bad operating system.
2.Hard disk failure or displacement
3.crashed or damaged hard disk
4.the system might be trying to boot from an external storage device like usb.
Posible Solutions:
1.Reinstall the operating system
2.check if the hard disk is well connected to the system.
3.Replace hard disk
4.Remove any external storage device connected to the system

Nice tips, Here's a guide: how to fix a computer that will not startup or boot . Hope it adds to the info above