Oh well there you go, despite the lack of beeps and despite the diagnostic lights I just changed the motherboard battery for lack of any better plan, and FIXED
Diagnostic lights 1 3 and 4 are on, so the user manual tells me helpfully that I can try restoring the OSbecause "A possible system board resource and/or hardware failure has occurred."Which makes little sense to me seeing as the bios hasn't displayed on the screen.   I'm suspecting mother board.  Any other ideas please
T3400 No beeps, screen blank, fan going full speed