It seems unlikely that people actually moved between your trees, but if they have, it is one very hard lesson learned, that at least for paid sites, it is wise to make backup gedcoms once in a while, in case something unexpected happens, so your data does not get lost.
IF you had backup gedcoms for both trees, which I realize you probably do not, but if you did, then I would take the bad data from one gedcom and import it into the other. It could be done either by hand or thru use of a family tree program on your computer.
Let us know if your grandmother actually disappeared, or it was simply a matter of your browser getting confused. This sort of confusion can happen if TP's cookies apply to all tabs and are not written in such a way that you can log in to two trees at the same time without a problem. The issue is not unique to TP - I think browsers in general seem to access one cookie file for all tabs together and do not keep separate cookie files for each tab.
The best way to avoid this sort of situation is to only log in to one of your trees at a time in any given browser, and open a different browser if you want to visit another tree here, as that will keep it's own separate cookie file. I do NOT mean a second session of the same browser, I mean two different browsers like Chrome and Firefox, or Safari and Edge. One tree per browser at a time.