I took it to the Geek Squad at Best Buy and they fixed the problem by removing and reinserting the RAM card, which rebooted it. That fixed the problem. They said my computer was in a neverending loop, probably looking in vain for an update that was never there, they said. Thanks to everyone who responded.
Teddy Bear
Favorite Answer
first you need to check that your cable connection is firmly plugged in. If this is not the problem, it could be a worn out or damaged cable and needs to be replaced. If the problem persists, the problem could lie with your video card. You will need to shut down and unplug your pc completely and open the box to check that your video card is firmly inserted (Note that opening up your box yourself will void the warranty. Also be sure that you use an anti-static wrist strap when working inside a pc). If you've tried removing the video card and re-inserting it and the problem persists, then you most probably need to replace the video card. If after changing the video card the problem persists then it could be a power supply or motherboard problem.
James
Check whether the monitor is connected properly to the VGA and DVI connector. Also check the cable connection.
Anonymous
Check the cable(s) to the monitor. Pull them out and plug them in again.
Scott
Graphics card.
Alex
is it a mac?