I've checked the main power cable and that is working fine; I checked that everything is plugged in at the back ok.
I even checked that the small lead that goes from the power button on the front of the computer to the motherboard is in ok. I tried putting an old computer case along side the malfunctioning one and connected its 'reset switch' to the 'power on' pin on the malfunctioning motherboard (this old computer case didn't have a 'power switch' wire) but it didn't activate the computer. I don't know what else to try.
What would be the most likely cause for the computer not going on? The motherboard is an Asus A7N8X-X. I'm not sure if you need more info, as the problem seems fundamental.
Thanks for your help.