It seems that the “EQCurves.xml” file in Audacity’s folder for application data has become corrupted.
The simplest solution is to delete that EQCurves.xml file then Audacity will use the default curves in the “EQDefaultCurves.xml” file that should be in the Audacity installation folder.
So navigate to UsersAppDataRoamingAudacity . If you cannot see that folder, show hidden files and folders. As soon as you delete EQCurves.xml in Audacity’s folder for application data, Equalization will show its default curves again.