It could be a number of things.
1. Check all your connections and make sure they’re secure.
2. It could be a loose wire inside the monitor.
3. If you have an LCD monitor, it could be the backlight.
4. It could be a problem with the power supply.
5. It could be a problem with the video card.
6. It could be a problem with the monitor itself.
If you can’t figure it out, it’s best to take it to a professional.