How can I get my iMac to be the primary screen again?

Hi everyone!!

A couple days ago I was watching a movie on my tv, which is my ''external monitor" for my iMac. I used VLC and it was full screen on my tv, I never had any problems with it. After the movie ended, I couldn't get my iMac screen to work. I restarted, and it flashes a white screen and it does make the apple startup sound. Also everything seems to be working just fine. When I hook it up to my TV, I can use my tv as the ''main monitor" but my iMacs screen remains black. (I have it in mirroring mode now) When I restart by pressing the on and off switch, I do get a flash of my current screen, but it fades to black quite soon after. Even if I wake my iMac up, it shows me the log in screen for about half a secodn and also fades to black. So it's not the screen itself that has any issues.

I think I accidentally clicked something in the display settings, but I have no idea how I can get my iMacs screen to work again. Does anybody know how to get my iMac back to functioning as a primary monitor? When I unplug the thunderbolt cable, it doesn't switch back either.

I already tried dragging the apple top bar from one screen to another, but it doesn't work. When I reset the PRAM, it does restart, but right after I hear the second chime the screen still remains black and it boots like it did before, still no functioning iMac screen.

Thanks in advance!