Why my system showing opertaing system not found

Boot from your install disc and re-install your OS.

Here's how to do it. When your computer boots,it will say something like press F1 to enter setup. Then find something like boot order, and use the arrow keys to change the value to optical or dvd drive. Then exit the BIOS and put your install disc in and reboot. Then,follow the on-screen instructions. I don't know what OS your using or I would be able to give you step by step instructions.