if you have moved files around, which is sounds like you may have done, then make sure you didn’t break the links to the library.
If that is the case, the easiest thing to do is to put the linked character in the correct spot and reopen your animation file. BUT MAKE SURE YOU DON’T SAVE THE FAULTY ANIMATION FILE BEFORE YOU MOVED THE LIBRARY FILE BACK.
That bit is in capitals because it is important. After you fixed the broken location, feel free to save your file then but if you save a broken file, it stays broken.
