@erichinkle most common issue would be a failed hard drive.
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On startup hit the F2 button (may take a couple of times) to enter the BIOS. Once you are in the BIOS check for a boot devices. See if your computer can still find your HDD.
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You could also try and set the boot order to USB and get a bootable USB drive. Boot from that and try to access your HDD. I use Puppy Linux Live CD installed on a USB flash drive and use it to gain access.