Most computer problems can be solved with a little bit of troubleshooting and an intuitive mind. When you can't see the problem because your computer monitor won't come on, the problem is a little more complicated and seems a bit more urgent. Fortunately, there are only a few possible causes of the problem, so troubleshooting shouldn't take too long.
Check the power. Make sure the computer and monitor are both plugged in. Try connecting them both to another outlet.
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Open the computer case by removing the screws from the case door. The door is screwed into the back of the case, and after unscrewing it should slide right out. Disconnect the monitor from the video card and then unscrew the video card from the case. Pull the card out and reinsert it. Connect the monitor and try turning on the computer again.
Make sure nothing else inside the computer is loose or not connected while the case is open. This is usually a problem only if you have recently been working in the computer and have not put the memory or central processing unit in completely.
Connect another monitor to the computer to determine if your monitor has gone bad.
Replace the video card if you hear noises when you hit the start button that sound like the fan starting or any kind of beeping. If all else has failed, the most likely problem is that the video card has gone bad and requires replacing. If you hear no noises and nothing seems to happen, then you most likely need to replace the power supply.