I have been editing a large collection of photos for the last month in Lightroom Classic. Today, when I opened Lightroom Classic, I discovered that all of the 4,000+ photos have reverted to earlier versions. The history for each photo now says "From LR mobile" as the most recent entry. I have never even used LR mobile. I also have been getting syncing messages lately, which may be related. Any thoughts as to what is happening and how I can revert back to my current edits and ranking without having to manually go back through all of the photos. If it is relevant, I am running Lightroom Classic on MacOS High Sierra. Many thanks.
This sounds like you had saved a copy (Back Up) of your Catalog and when you re-entered your catalog you inadvertently opened the previous Back up catalog, not your primary Catalog. What you need to do is to find the name of your catalog and then do a search for that name. If you find two, Bingo.
I suggest you give your primary one a new name AND set your LRC to ask "which Catalog do you want to open" as a default.