This is a very common problem that can have a few different causes. First, check all of the connections to make sure that they are secure and tight. If they all seem to be in order, then the problem is likely with the monitor itself. Try unplugging it from the power source and then plugging it back in. If that doesn’t work, then you may need to replace the monitor.
There are a few other things that could be causing the problem, such as a loose cable or an issue with the graphics card. If you have checked all of the connections and tried unplugging and replugging the monitor, then you may need to consult a professional to diagnose and fix the problem.