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If your default Boot Device is missing, it can mean that your hard drive is failing.
However, it can also mean a lot of other things.
How many hard drives do you have in this computer?
okay, that narrows it down considerably, and eliminates a DUAL boot scenario (most likely).
Do you know if you have any PARTITIONS on the hard drive?
How OLD is this computer?
When did this issue first happen? And did you make any changes to the computer prio to this problem? Like adding any hardware....like a PCI card....or RAM.....or anything like that?
Did you INSTALL any SOFTWARE prior to this issue?