displaying CPU details on screen, but nothing - keyboard not responsive, CPU fan and internal fans and disks whirring, but nothing else.
turned off, turned back on.
same. single beep on POST, suggests all ok.
took cover off, tried reseating cards/memory
tried unplugging/replugging HDs
unplugged all USB leads from back
bingo - it's up and running again.
another note- the heatsink atop the onboard graphics is fairly warm/hot to the touch, but the PC has been off all night...
I don't use the onboard graphics, as I've got a separate graphics card.