This comes up so many times…and the Windows user solution always seems to be re-install Gimp which 99% of the time changes nothing.
The reason. All the Gimp user settings are held in a folder, created first time Gimp runs and crucially remains between Gimp installations. Works that way for users to migrate to a Gimp update without losing their settings.
You can find the location of that folder in Gimp Preferences by looking at a folder entry but for a regular Windows installation it is C:\Users"yourname"\AppData\Roaming\GIMP\2.10\ and Appdata is a hidden folder so ‘unhide’ it. see: View hidden files and folders in Windows - Microsoft Support
If you delete that …\Gimp\2.10 folder and start Gimp, a new default User profile is created where everything works as expected.
.gimp that AdmFubar referred = C:\Users \ “yourname”\AppData\Roaming\GIMP\2.10\ Instead of deleting you could rename, it has the same effect…but if it is a new installation nothing lost by deleting.