Do you have a laptop or a desktop?
If it's a desktop, then make sure the connect between your monitor and PC is plugged in and screwed in tightly at both ends. If this doesn't fix it then your monitor could just be going bad.
If it's a laptop then you have a bigger problem. If a laptop monitor is causing you issues then it could be a loose wire within, and without the correc tools and training it's difficult to try and fix the issue yourself by taking apart a laptop.
On either a laptop or a desktop it could be a motherboard issue or more specifically a video card\video chip issue.