It's either the supply or the motherboard. Since troubleshooting costs more than either part we swap in one, then the other.
Of course an old CMOS BATTERY has been known to do this too. Cheaper than the other parts so tell how old it is. If 2 or more years try it since it's only a few dollars and worth the risk.
Keep in mind that I'm writing from just what was told, which was quite slim.
Hi, i have a Sony Desktop PC, it won't boot up though, when i turn the power button on, the PC will turn on, the fans will spin and everything works, the CD drives will eject, but the moniter stays on standby, the moniter and cables all work as i tested them.
I have already tested the graphics card and that hasn't changed anything, i'm not sure i'm getting the BIOS start up beep.
I don't know if this will change anything but when i put in the power cable into the back of the PC sometimes it will turn the PC on by its self. But the same problem is there it just turns on.