Try re-seating the video cable. Try another monitor on this computer if you have one available. Try this monitor on another computer. That will narrow down where the problem lies. It’s either the video card or the monitor which is bad. Of course if either is under warranty you may get a replacement, if not get your credit card ready Good luck to you!
I would also look at the cable connection and see if there are any bent pins. This has caused issues for me in the past. Just be careful rebending the pins if any are bent. They are fragile and may snap off.