I have optiplex 780, On stsrt up the diagonistic lights 2,3 and 4 keep on blinking and no display comes on the monitor. According to Dell 780 manuals, a possible USB error may have occured and re connecting all cables should solve the problem. I have tried this solution many times but the problem remains there. What may be wrong ?? Please tell.

Shut down the computer, disconnect ALL USB connections front and back.

1. Plug in just the keyboard USB cable and start the computer. If it does not work, try moving the USB plug for the keyboard and trying again. If you can't find ANY USB port that works, then your motherboard is probably fried.
2. Once the computer comes up with just the keyboard, plug in the mouse and wait for it to work (should be almost immediately). If it does not work in one USB port,try another until it works... wait several seconds after plugging it in before deciding whether it works or not.
3. Once you have a working keyboard and mouse, begin plugging in any other USB devices, one at a time and verifying that each device works. As with the keyboard and mouse, if it does not work in one port, try another.
   1. Note that some devices may take several seconds or up to a minute to fully be discovered and set up and you may have to respond to something on the screen.