My pictures are stored on an external hard drive. My LR catalog is on my computer (a Macbook Pro). I upgraded from LR 5 to 6 about a month ago. No problems. I use LR a lot, and when I quit it, I always back up the catalog.
For reasons that I don't at all understand, Lightroom is suddenly not showing any photo folders dating from February 2016. That is, photos that I imported before then are all present and accounted for. But all the photos that I imported (and edited!) from February to the present are missing from the Library. I checked the hard drive, and the picture files are still there.
Obviously I do not want to re-import the missing photos (thousands of them) and lose all my edits. And I can't ask Lightroom to find the missing photos because they don't appear in the Library. I am therefore at a loss as to what to do.
Sounds to me that one possibility is you accidentally opened the wrong catalog file. Go to File->Open Recent and select each catalog file shown there until you find the catalog you want.
I opened the most recent catalog file. Maybe that's the problem (for some mysterious reason). I will open an earlier version and see if my folders reappear.
I will never be able to figure out what happened to the most recent version of the catalog, but I opened an earlier version and all the missing folders reappeared. I am very relieved. I have only lost a handful of edits.