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.
@voracread I think ~/.config is the standard config file location on Linux (for apps that follow XDG standards) so it should also have config files for non-kde apps
It’s the standard location for all apps (actually it can be overridden by environment variables and ~/.config is the default value). However like many things in the Linux world it’s not enforced. Some apps (especially console utilities) don’t respect it but most use it.