U2419H, display only visible in portrait mode?

I’ve been using my Dell Monitor U2419H for the past 3-4 years. Currently I am using it with my Apple Mac Mini. For the past 4-5 days I’ve noticed a problem where the display keeps coming on and off. When I rotate the monitor in portrait or vertical mode, the display is visible in these modes, but when I rotate the monitor to landscape or horizontal mode, which is how I usually use it, the display goes off. This issue has been happening for several days now. Could it be a faulty power cable? I’ve tried removing the power cable and reattaching it firmly, but that didn’t make any difference. The display is visible only in portrait mode, but when I rotate the monitor to horizontal mode, the display goes off. What could be the reason for this problem?