Big Monitor problem, what can I do?

It's like that, first I want to tell the prehistory… I had started Fortnite in a Custom Resolution (1440x1080), while Fortnite was open in the background I just deleted the RESOLUTION 1440x1080 in my AMD setting while the Fortnite was still running in that resolution was… Since the monitor just does not work, he runs on DP and there's only DISPLAYPORT NO SIGNAL… But when I connect my 2nd screen (DVI) my actual suddenly works as a 2t monitor again, but as soon as I finish the other again is NO SIGNAl again… I have no idea what I can do and ask for help as soon as possible

Plug the monitor cable (So DisplayPort) into another port on the video card.

I'm sure your video card has at least 3 different DisplayPort holes for the cable.