tmux session lost in unknown pts, cause and possible solution?

I asked this question on the Unix/Linux stackoverflow, but a suggestion is not forthcomming and people ask if I can run commands with root access, so perhaps WebFaction-support is more relevant.

Question:I was trying to re-attach to a long-running tmux session to check up on a python web-application. However tmux attach claims that there is no running session, and ps shows a tmux process (first line), but with a question mark instead of the pts number.

What does this mean---is this tmux session permanently lost, and what could have caused it? Is there still a way to look at the current state of the python process, spawned in the tmux session and running in pts/19 (second line)?

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[mhermans@web314 ~]$ ps -ef | grep mhermans
mhermans 16709     1  0 Mar04 ?        00:26:32 tmux
mhermans  8526 16710  0 Mar04 pts/19   00:20:04 python2.7 webapp.py
root      9985  6671  0 10:18 ?        00:00:00 sshd: mhermans [priv]
mhermans 10028  9985  0 10:18 ?        00:00:00 sshd: mhermans@pts/16
mhermans 10030 10028  0 10:18 pts/16   00:00:00 -bash
mhermans 16247 10030  6 10:28 pts/16   00:00:00 ps -ef
mhermans 16276 10030  0 10:28 pts/16   00:00:00 grep mhermans
mhermans 16710 16709  0 Mar04 pts/19   00:00:00 -bash
mhermans 16777 16709  0 Mar04 pts/21   00:00:00 -bash