Went for lunch yesterday and when I came back to my PC it was off (running for 6 hours as usual). Tried to reboot but it would not start. checked power chord, switched it off at back, then turned back on and it rebooted so far and then cut out again. Switched it off again and back on at the back and this time it rebooted ok and ran ok for at least another 8 hours.
This morning I booted up and as I went to log on it cut out again and will not power up at all.
I've had it apart and there is no obvious damage or loose connections.
When I turn it on at the back, the power supply fan initially spins and then stops, which I believe happens normally but then nothing.
So far I have found 4V on the motherboard connector.