I replaced the power supply in my Dimension 9100 today. Took out the old one and connected everything to the new one. Everything worked perfectly.
I then tidied the cables screwed everything shut again etc and now when I switch on the PC the fans all start, the LEDs come on, the disks and DVDs etc make the usual startup noises but all I get on screen is a flashing cursor in the top left corner of the screen. I don't get any beeps and the BIOS screen isn't displayed.
I assumed I must have knocked something loose when shutting the case back up so I opened it up and made sure everything was still connected and firmly seated but it hasn't made any difference.