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This is NOT a good thing to have happen....as it usually indicates a hardware failure.
Because the Operating System is the Core of the computer software. Without it....the computer is basically useless. If the Basic I/O software (BIOS) cant find the OS on the hard drive....it usually means a few things.
1- The hard drive has failed, and therefore the OS is not found, because the OS lives on the hard drive.
2- The hard drive has been removed form the list of bootable devices within the BIOS. (Virus or possible Malware issue) But this is unlikely.
3- Motherboard failure, not seeing the connecting to the hard drive. Also not likely.
4- Power supply is not powering the hard drive, and keeps it from working.
There are probably more possibilities....but the most obvious should be tested first.
can you boot from a DISC? DO you have a Windows DVD disc? Can you boot form it?
If you can, you can run some diagnostics on your computer with it.
But the best thing is to use a diagnostic boot CD disc, called ULTIMATE BOOT CD.
But you have to download it, then burn to disc, or order one.