My desktop computer is not booting, operating system not found

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Common causes I run into are:

  1. The drive that had a working OS now does not.
  2. The OS drive has failed. Bad drive, sectors, disconnected or just missing!
  3. The cable to the OS drive is unplugged.
  4. The port on the motherboard is damaged.
  5. The drive is connected to the wrong motherboard port.
  6. The BIOS was changed from one mode to another.

I find many owners today do not know a thing about their PC and have to work down the usual list and watch out for clues along the way.

List of possible reasons in addition to the abovementioned also include:
Damaged hard drive
BIOS does not detect the hard drive
Corrupt Master Boot Record (MBR)
You have a CD disc in and your BIOS is set to boot from CD
You have left a thumb drive in a USB slot and its trying to boot off that

If its none of the above, maybe your bootsector has been damaged.

BOOTREC.EXE is a WinRE tool (available in Windows Vista and later releases of the
Microsoft OS) used for troubleshooting this type of error.

You possibly need to write a new boot Sector
use as follows(in WinRE):

Bootrec /fixboot

For detailed help see:
Click Here for Microsoft site

@marcza - this assumes he is trying to boot Windows. He doesn't say.

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