I have always worked with 2 LrC catalogues, one in my Mac, which only contains the latest assignments, and then another one on an external drive (with a duplicate back up) which contains a selection of photos of ALL my assignments. This second catalogue contains many collections. I worked in this way for many years.
Today, after months of not using LrC, I plugged my external drive, opened the LrC catalogue, and realised that on the left sidebar, under Folders, where it would normally show Macintosh HD, and MySel-1Tb01 (my external drive), the external drive wouldn't show, and all the collections that had been organising for years, didn't show either.
It basically only showed my Mac Catalogue with my latest assignments and the collections that I created on my Mac catalogue. (No external drive catalogue photos not its collections).
On the left sidebar, next to Folder, I clicked the plus sign and clicked on Add Folder, and I added my external drive folder (where I have all my photos). It took a long time, but it added the photos and its folders, but it didn't add the collections, that had been "saved" in the external drive catalogue that I've had for years, and I have always kept updated.
Is there a way to recover the collections? Or to open my external drive catalogue as it was? Even the back up from the last date when it worked didn't contain the external drive catalogue (and its collections). In both cases, it opened my Mac catalogue.
I'm afraid I am not able to confirm if that was the only reason. I ended up using an old back up, but I (and the people from Adobe that tried to help me remotely) still think there's a problem that we haven't been able to figure out, since my catalogue had been fine for years, and I had a fairly methodical way to operate it.
Still, things are fine and didn't lose that much after all.