From reading about this problem on this and other forums, I know loss of power can be caused by any number of things, so I will make it easier for those of you more in the know than myself to figure out by saying specifically what triggered it - a photo frame fell down the back of the tower and lodged on the cables. It would have struck the power cable first. Screen froze and now keyboard monitor and mouse aren't receiving power.
What I can say is
1. No beep on start up
2. Fans on psu and CPU work fine
3. Tried CMOS reset manually
4. Tried unplugging everything, to reset electronics
5. Unplugged mobo from psu
6. Changed power cables to both monitor and tower
7. Changed monitor signal cable
8. Graphics card is on board
Also, for years this machine has had problems starting up - almost every time, DOS will appear on screen and then freeze with the letters shaking and distorted. Resetting the tower a few times always sorts this.
I can provide some technical specs if it helps, but this'll have to be what I can deduce from looking at the components.