In the /home directory, there should be a hidden directory called .kde which mostly contains KDE settings for your user. Rename it and check. A new one might get created with default settings during next login.
Caution: I am no expert and consider all backup options before doing this because it may force you to re-install.
Edit 1: based on additional searching due to comment below, it was ~/.config for KDE 5. I have no idea of what it is for KDE 6. May be you will have to go through source code to find.