Do you have diagnostic LEDs on the motherboard ?
If you have them (and I think you should have them), then you can remove the correct side cover of the PC case and look at the motherboard. One of the LEDs should be shining bright red. Depending on which LED is shining red, you can take corrective action.
In most of the cases, corrective action is to reseat the troublesome card or RAM stick or to press the mem-OK button. Your motherboard manual may come handy.