After a power outage, the motherboard connected to the video card needs to be reset. Unplug the computer and push the power button for about a minute before you plug it back in. If that doesn’t work, you may need a new monitor.
Wow, good tip, thanks, bookmark in case needed for future.
Before buying a new monitor, try it out with a different monitor - borrow one from a friend - the test will only take a few minutes. If the problem still exists with another monitor, it is the video card or video on the motherboard that is at fault.