I have just upgraded to v. 10 of LRC, and all of my collections (lots of collections!) have disappeared—with the exception of those photos synced with LR on my iPad. Is there a way to regain all of those lost collections?
You've likely tripped over a bug in Mac LR 10. To work around it, expand the Collections panel and then restart LR. Or click + in the Collections panel header and change Sort By. Or create a new collection.
Please add your constructive opinion to the bug report: