Hard to say why that would be. Lets focus on what to do to get it fixed. I'd unplug that monitor both power and from the computer and let it sit a couple minutes to discharge. Hit your power button on the monitor while it is unplugged. That should discharge it. Unplug your video cable at both ends computer and at monitor. Do this with the computer off. Reconnect it all and restart and see what happens. If this does not work. Go through your monitor settings and see if there is a reset. If that doesn't help you need to give the requested information for asking a question before anyone can begin to try to help.