As above introduced, an incompatible Active partition or MBR in the NOT active partition would also cause operating system not found issue. Systemdiscs (it is not a free product)gives an friendly user interface to fix this error. Of course you need another computer to download this software and Burn or Make a live bootable usb stick.
Learn How to create a Bootable USB stick?
Now, follow the steps below to do what you need.
After you have created a bootable cd or bootable usb stick plug them in the computer with error and reboot the computer. After the computer boots up, you will see something like on the pictures below.
Follow the onscreen steps, and you will get the following screen.
Click the Automated Repair.
Select the operating system you are trying to fix and click on Automated Repair.
The repair process will then start.The Cd will then check for dozen of known issues, scan the machine for corruption and damaged files and reconstruct the bootloader. Any errors found on the process will be repaired automatically.
Once the process is finished you will be asked to eject the Cd and reboot the computer. After rebooting your computer should boot up and start normally.