Sometimes the power switch works sometimes it doesn't .I've disconnected the switch and touched the leads together. I've replaced both the power supply and the motherboard to no avail. The video card doesn't seem to work either( no picture) although removing it doesn't change anything, so its probably a separate problem.

Any guesses would be appreciated.Thanks

How old is the machine?

I had this issue was back in about 95 or 96 on an old Olivetti pc (dont know if they even make them anymore lol). You would hit the power button and the disk and power lights would come on, and stay on, but it would just sit there with a black screen.

I never fixed the problem. Sometimes it would work, other times it wouldnt.

Bad/Dying PSU maybe?