losing list of read messages

So I somehow lost the list of read messages in one of the newsgroups I
read. I went there and all messages were marked as unread, when I had
read every message for the past couple of months. It only happened to
one newsgroup, and I can't figure out what happened. It's really
annoying because I'll have to reread old messages to find older ones I
haven't read. Any idea how I did this? It's fully possible I typed some
weird key combination in tin, or even messed up a file in ~/.tin. I'd
like to recover my list of read messages for this newsgroup, or at least
be sure it won't happen again.