Hello,
I'm not sure, if this question belongs here, but I was told to try to ask it here, so here I am.
I upgraded all packages via "Discover" and rebooted. Now, my second screen doesn't get detected.
The lockscreen does show on both screens, but once I log in, my second screen doesn't get a signal anymore.

I am using nvidia-drivers-450, I had issues with the nvidia drivers before, but always got it to work after some time. Now I have been trying to get it to work again, but I can't seem to get it to work.

Any help would be very much appreciated!

How about telling us the important stuff: your distribution and exact Plasma version, and did it work before the update?
Also how is the screen connected?