I set a certain window to "no title bar and frame". Now I want to undo that setting. Normally, I would right-click on the top bar, and choose the windows settings I want, but the top bar is now hidden.
Just go to: System Settings/Window Management/Windows Rules
You will see a list of all windows that have special rules. Select the desired rule, and click on "Modify".
Another, probably easier solution is to use the keyboard shortcut alt+f3, which should bring out the top bar menu. This solution is from bac0n's comment on the question.
This question was asked and answered already, albeit on a different site: https://forum.kde.org/viewtopic.php?f=66&t=109268
Just go to: System Settings/Window Management/Windows Rules
You will see a list of all windows that have special rules. Select the desired rule, and click on "Modify".
Another, probably easier solution is to use the keyboard shortcut alt+f3, which should bring out the top bar menu. This solution is from bac0n's comment on the question.