Occasionally when I press the on button on my PC the fans will switch on (as will the power light) and the hard drives and CD drives spin up but nothing else happens. The BIOS doesn't make its usual single short beep, the hard drive/CD drive lights don't come on and clearly no power is getting to the video card as the monitor doesn't switch itself on as normal (switching it on manually has no effect as it just goes straight to standby since it is receiving no input from the the PC).
To rectify the problem, I have to power off the PC (by holding down the button for four seconds) and try again. Sometimes it works properly again immediately, other times I have to switch it on and off several times and sometimes I have to come back to the PC after 15 minutes and it will work again.
I should mention that this is a very infrequent problem (maybe only once every six weeks or so) but it is very frustrating when it does happen. The time has come for me to try and fix it as I am worried something on my PC is on the way out.
Is this a sign that my motherboard or power supply unit is failing? That is the only explanation I can think of but would be grateful for some confirmation.