The computer comes on, but nothing appears on your monitor.

In other words, Windows never shows up. You may have a monitor problem. Try using another monitor you know is good on the computer and see if anything shows up on the screen. If the second monitor works, the first one is bad. Monitors are not worth repairing. Just buy a new one. Never open the back of a monitor to fix it. The capacitors inside monitors store electricity. You could be injured or even killed. If the screen is dark, it could be a video card problem. First, find the video card. This is a circuit board that fits into a slot in the motherboard. The cable from the monitor connects to the VGA (video graphics adapter) port, which sticks out through the back of the computer. If the VGA port is part of the motherboard, the video is built in. You can't fix that. Otherwise, it will be part of the video card. Assuming you have a separate card, be sure it is firmly seated. The front end of the card can rise out of the slot inadvertently when the back end is screwed down to the computer frame. If you have a computer that is working perfectly, turn it off and remove the video card. Put the card that works in the problem computer. If the system works, you need a new card. If you don't have an extra card to test your system, buy a cheap one ($30-50). If it doesn't solve the problem, take it back. You can spend hundreds of dollars on a video card. But if you're running business applications and surfing the Web, buy on price. The expensive stuff is for serious gamers.