A cable that's a bit loose can be connected sometimes, then when a tiny little vibration or something shakes it even just a little bit then it disconnects. Just check anyways to be sure
After chkdsk has done it's thing I'l try nudging or moving the computer and see if it dies
and if I come to no solution after everything everyone's suggested I'l try opening it up and making sure it's plugged in
Speaking of, chkdsk just finished and it exited itself, what did that do?