SUMMARY
Hello,
Excuse me if I don't explain well the problem but I really don't know how to describe it. It happens to me for second time already. I just turn off my monitor, do something for some time and when I turn it on again, everything is unusable. There is no windows title bars, the panel is unusable. It shows some items but they are not real. These items can't be closed or refreshed. Here's a screenshot:

https://i.imgur.com/kUAvIkN.png

As you can see the desktop effects are gone too. When I click on panel items they are unresponsive. When I open new apps or something, the panel items doesn't change and the list stays the same. Alt+F2 doesn't work. The only way to start a program is via Kick off menu via Meta (Win) key but there are no way to close it for it is not listed on the panel end there isn't a close button. 

I started the Compositor menu and tried to change rendering backends, scale methods etc. None of the options are helped. Tried to restart Plasma via 'kquitapp5 plasmashell && kstart5 plasmashell' to no avail. It actaully closed all open apps but they was yet listed in the panel:

https://i.imgur.com/TlhgDjk.png

Alt+Tab doesn't work too. The only way is reboot via TTY. I have all powersaving options disabled because if they are active they trigger another problem so my system don't go to sleep, hibernate, turn off monitor or something, it's just working and I don't know why it depends on monitor' state.

STEPS TO REPRODUCE
1. Turn my monitor off.
2. Turn the monitor after some time - e.g. an hour.
3. 

OBSERVED RESULT
Everything is broken, there are no controls and titlebars, the panel is unusable.

EXPECTED RESULT
To work properly.

SOFTWARE/OS VERSIONS
Linux/KDE Plasma: Arch KDE
KDE Plasma Version: 5.18.3
KDE Frameworks Version: 5.67.0
Qt Version: 5.14.1