I am working on a computer that had the same problem that it had when I fixed it a month ago. The power came on ok but the monitor was blank. I tried with a different monitor and a different outlet still the same problem. I decided then it was the motherboard and swapped it with a new one. That worked for a few weeks but now the same problem:
The computer has an ATHLON CPU and now has a Asus(tek) A7V-VE motherboard. I notice that the fan does not come on but I think that it is temperature controlled.
I tried it with a diagnostic card which shows FFFF which I think means either the CPU or Mobo is dead.
Any ideas welcome.