I clicked the “New” button in the Library window (I wanted to add a new material and new looked like what I needed), but it appears that the New button create a new library. So now my old library is lost. I looked in the directory under “File/Open Perfs Folder” but did not see any CLB files there. I searched by C: drive and did not find a CLB file.
The library will be wherever you originally put it - Clicking ‘New’ doesn’t erase the old one, it just makes a new library file for you. Click ‘Save as’ in the Library window and it should bring up the last path you saved a library to. You might find the previous one there.