1. Check that no wires are lose. During the changing of power
supply, you could have accidentally loosen some cables or some
things.
2. Take out your RAM and put it back again. Make sure there is a
click sound. This click sound will confirm that it is put back
properly.
3. Check your monitor cable. Make sure it is connected properly. If
not, you will get this error too. Also, if your graphics card is an
external one, make sure it slotted in properly. Tighten the screws
if necessary, but don't make it too tight, you will spoil it.
4. Check your power supply cable. Make sure it is connected
properly to the motherboard. The pins must match each other.
