I was having fun playing for a while, but then the game un-purchased all sets, took my BP away, and started displaying the main menu as the background for every screen. Even after deleting and redownloading the software, this persists. Does anyone have a fix? I really want to keep playing.

Sorry to hear that. Something like this never happened to me. It sounds to me like the save files/settings are corrupt.

You could try to reset these files by deleting or renaming the "TheBricksperience" folder in "c:\users\[your username]\AppData\Local\". The "AppData" folder is hidden by default.