Can't text one of my contacts...

3jam has been working fine for a while but the other night, one of my contacts sent me a text and it said at the end of it " (Contact's name) has left this 3jam" and now I can't text them from 3jam, and they can't text me. So what's wrong? I'm also having the Code 21 problem if that matters, but someone else already asked about that, so I won't bother with that one yet.