This problem sounds very familiar. And I think Bobscrachy has hit the nail on the head.
Sitting on my table are 2 PCs. One is for my kids, and the other is all mine. The problem is that we share the same monitor, so from continually swapping the monitor over, I - or my kids - damaged the connection on my mainboard, which resulted in the same boot problem.
Fortunately it was still under warranty
John