tmux keeps track of active sessions in a file, on my system this is in /tmp/tmux-1000/default. Recently I manually cleared everything in the /tmp directory, removing this session file.
Normally, you can use tmux attach to reattach to a session. Active sessions can be listed using tmux ls. Because this file is gone, tmux doesn't know about any active sessions:
$ tmux attach
no sessions
$ tmux ls
error connecting to /tmp/tmux-1000/default (No such file or directory)
Using top I can verify tmux is still running, along with the applications I had opened in that session.
How can I reattach to this session, after I've removed the session file?