It started with an electrical burning smell, then nothing - no fans, no lights.
Tried a new PSU - same problem, so tried disconnecting different power leads in turn. When 4-pin ATX 12V lead was disconnected, fans worked and power light came on, but nothing else. Same with old PSU, except that PSU fan didn't spin at all, even when CPU fan did.
My guess is that part of motherboard circuitry is burnt out.