Computer lights and fans turn on but nothing happens to the screen

Computer lights and fans turn on but nothing happens to the screen

After replacing my old power supply I put an old 250w PSU in until I get a new one. Before I put everything back I I tested to see if it would work and it did. But once I tried connecting a monitor to it I realized that there was nothing on the display. I had tried everything of my ability from onboard graphics to a different video cards, nothing was working.