Two suggestions: scroll down the selections in parts sometimes the search moves to the bottom of the list. If that isn't it then try deleting the user directories (note you will lose all your sketches and imported parts doing this so back them up first or just move the directory aside and replace it). I assume Sierra means this is a Mac (as there have been lots of these types of problem on Windows but not so many on the Mac)? Note that reinstalling Fritzing does not reinstall (or change) the user data (your sketches and parts) or temp directories and sometimes that is what gets corrupted. I think on a Mac the user directories are in a . file in your home directory (I'm a Windows user).
Peter