I moved all my image files from internal to an external drive, intending to leave them there while using LR on a new pc. The int->ext move was done properly according to instructions found online and everything seemed fine at that stage.
Having connected the ext drive to the new pc, installed LR and put the catalog onto the int drive, I double-clicked on the catalog file and LR opened up with the Library module looking as expected, with question marks on the folders.
However, after I went through the Find Missing Folder procedure on one of them, the others disappeared. Unfortunately, the missing ones contain the vast majority of the image files. (The actual folders are there on the ext drive, they're just not showing up in the Folders pane of the Library module. The number against All Photographs in the Catalog pane looks about right.) Where do I go from here?