Suddenly a large # of image folders, disappeared, that were previously imported, but can be seen after using the import button, as if they need to be imported again. How do I recover these, re-import? I use LIghtroom 5.7.1, and have a pc, with windows 10.
My guess is that you accidentally opened the wrong catalog file. Try selecting each catalog file shown under File->Open Recent adn see if the folders of interest have been imported into that catalog.