The unlink just happened to me. Restarting doesn't fix it.
In an odd sort of way, I may have "caused" the problem. I'm working with a very small test RM database. From the RM side of the house, I deleted all the people in the database and then added a new person. Upon going to the ancestry database, the tree was unlinked. It's still there on the ancestry side of the house. Just to move on, I'm going to delete the tree from the ancestry side of the house and then use RM to upload my one person data base to a new database at ancestry.
The reason that I wonder if I might have "caused" the problem is that the empty database may not be a situation that has been programmed for with an RM database that is linked to ancestry. Indeed, I had a hard time deleting the last person out of my little test database. It was like it was ignoring my instructions to delete the person. But I kept just sort of playing with it, trying several things over and over again to try to delete the person (one of the definitions of insanity!). I'm not sure what I finally did, but something I did finally deleted the last person. It was subsequent to finally deleting the last person that I discovered that the database was unlinked from ancestry.