Mine does this from time to time, i just stick the windoze cd in and do an installation repair when it does.
Once the disk is in the drive, the PC should try and boot from the CD already, but if not just go into bios and change the boot order around so it tries to boot from the CD rom drive first.
Once it has started reading from the CD just follow the options and repair it, you shouldnt lose any of your files either, well, i didnt anyway.
This always fixes it for me for a good few months, but my pc is totally knackered and old and really needs putting in the bin TBH
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