The ideal scenario is that all the active development and community has moved to some other fork; in that case, just put a big notice at the top of your README saying your repo is deprecated, with a pointer to the new central repo.
However, the usual case is that there's a dozen forks, all with one extra change or fix, all equally stale since their authors happened to fix their particular itch. Perhaps you could contact each of them (or at least, the biggest and most popular ones) and see if any were willing to be nominated Official Maintainer. If there's exactly one, they win. If there's more than one, put them in contact with one another and let them sort it out (much better than having them find one another by accident and letting them fight it out later). If there's zero volunteers, I guess you just have to post an 'abandoned' notice with no forwarding address.