Do a CMOS reset first, then boot your PC and check the LEDs on the mobo.
When you boot, before reaching POST, it will check for DRAM, CPU and VGA.
Start the test with just CPU + 1 stick of RAM at the recommended single RAM slot based on your manual. See if it boots.
If it doesn't boot to POST, it is likely to stuck at either CPU or DRAM check. The LED of the component check will hang there (ie. permanently lit) which will tell you which component check failed.
If it stuck at CPU then try reseating CPU and HSF and try again. Check for bent pins, burnt marks or any visible dmg. If still doesn't boot then prob CPU failure.
If it is RAM, try all your other RAM sticks on the same slot. If either one of your other RAM sticks boots, then those RAM sticks that doesn't boot are faulty.
If all RAM sticks doesn't boot on that slot, repeat the 1 RAM stick test on another RAM slot. If it boots, then the initial RAM lane you've tested is faulty.
If all RAM lanes doesn't boot, either all RAM lanes died or all your RAMs died. Can only be confirmed by bringing your RAMs to another working system to test to deduce whether it is RAMs or the RAM lanes.
If you past this step, test GPU. Plug it to the 1st PCI-E lane and boot. If it doesn't boot and the VGA LED stays lit, try plugging the GPU to the 2nd lane. If it doesn't boot, plug it to the 3rd lane (if only your default BIOS settings doesn't disable the 3rd lane). If all 3 lanes doesn't boot and stuck at VGA check then most likely your GPU is faulty (although there is a slight chance that all PCI-E lanes are faulty if you are that god damn suey).
If any of the lanes boots with your GPU then those lanes that doesn't boot are faulty.
