there are some reasons for Operating System not found
the hard disk is damaged;
the BIOS cannot detect the system installation hard drive;
system boot files not active anymore;
The MBR has been corrupted.
Thus, the solutions
could be one of them:
1. Check the BIOS settings. ensure IDE Configuration:Primary IDE Master, Primary IDE Slave, Secondary IDE Master, and Secondary IDE Master can detect the hard disk.
2.Create a bootable media and reboot from it, set boot order and restore OS.
3.Set the system partition active.
4.Rebuild MBR with AOMEI Partition Assistant.