The most common reason for this error is when your PC is trying to
boot from a
hard drive or
flash drive that is not properly configured to be booted from; in other words, it's trying to boot from a
non-bootable source. This also would apply to media on an
optical drive or
floppy drive that you're trying to boot from.
Other possible causes include corrupt and misconfigured files, hard drive and
operating system upgrade issues, corrupt hard drive
sectors, an outdated
BIOS, and damaged or loose
IDE cables.