Go back to BIOS setup.
Check under boot devices, if you're booting from the correct harddrive.
If there is no harddrive selected it will try boot from other devices, such as cd-drive,floppy.
Had similar problems, when setting my bios.Sometimes it would change the device list boot priority for my harddrives.And having 3 harddisks it picked one at random.Causing it not to boot.
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