The screen is black...

If it does turn on and you can see lights on the computer or hear the fan spinning:

For Desktop: After checking power cables
  1. Check your monitor is powered on.
  2. Make sure your video cable is plugged into not only the computer but the back of your monitor firmly.
  3. If both answers are yes, you can try pulling out the RAM cleaning it and plugging it back in.

For Laptop: After checking power cables
  1. Check the keyboard area for any power lights turning on.
  2. Unplug AC Adapter from the unit, remove the battery, wait 10 seconds, plug battery back into the unit.
  3. Try pulling out the RAM cleaning it and plugging it back in.