I have tried everything. Now I need everybody’s help with this problem I have. My Dell optiplex 790 wont turn on for some reason. I have installed a new power supply and did some troubleshooting. I pressed the diagnostic button on the back of the supply and everything lights up. My computer refuses to turn on for some reason. I have tried everything. I need help. My dell optiplex is only 3 years old. I got it from my uncle.