This morning my computer wouldn't turn on. Pressing the power button has no affect. When I turn off the switch at the back and push the power button after turning it back on it seems for about half a second that everything works: fans appear to start moving and the PSU light turns on.
This happens only for a brief moment (less than a second) and then everything turns back off. I can repeat this by turning off and on the switch at the back of the case or by unplugging the power cable out and plugging in again.
There is a small LED on the motherboard which is always on when power is connected (doesn't matter if the computer is on or off) and it is still on now.
I am trying to diagnose what is the cause of the problem PSU or motherboard. Any ideas?
Best Answer
Unplug everything you can possibly unplug, internally and externally, and remove all expansion cards and RAM. So, with nothing but the main ATX power header and CPU power header (and front panel and internal speaker connections attached - see comments) attached and no RAM, if the box STILL won't power on, you probably either have a power supply or motherboard problem (the odds of a failed CPU are nil unless you've been overclocking like crazy, and if you're savvy enough to do that you shouldn't be asking this question).
If it beeps at you, which it should if you try to power it on with no RAM, reattach stuff piece by piece (starting with the RAM) to see if you can isolate a problem.