If it isn't running then the socket doesn't matter because deleting it wouldn't kill tmux, it would just stop you attaching to it.
There is no way to find out what happened to it after the fact unless there is anything in your system logs showing it crashed or was killed.
You can run with -v to create logs to see if it happens again but they will be very big.
What tmux version are you using?
"ps -aef | grep tmux" does not show any process of tmux.
What do you mean by "the socket wasn't just deleted"? How can I check/confirm that?