This same thing happened to DW's Laptop a couple of years ago. I believe I used Memtest
to identify what had happened.
A most interesting thing happened this week to my Laptop. When I booted it up in the morning, the following message kept repeating:
PXE-E61: Media Test Failure, Check Cabling.
PXE-M0F: Exiting PXE ROM.
I took the "C" drive out and connected it to the Desktop machine and found it was readable and copied all (not that many actually) the files that had not been backed-up on to the Desktop machine. I, then, put the drive back into the Laptop... No joy.
While booting up I noticed that the F12 key would let me select the boot device. I clicked on that and noticed that my "C" drive was fifth on the list and the "D" drive was first. I told it to boot to the "C" drive and the machine booted up as expected. For some reason the BIOS was changed, switching the boot sequence -- I had never heard of such a thing.
The Error Message, BTW, was telling me that the Machine could not find the Network Boot Device, which was third on the list after the CD/DVD drive.