On the previous day, I accidently removed /tmp/tmux-1000/ directory. At first, there seems no problem with Tmux. But when I created another SSH connection, I saw this error message:

$ tmux attach
no sessions
$ tmux list-sessions
failed to connect to server

But the Tmux prosesses were still there:

$ ps -ef | grep tmux
59277 16305  0 May06 pts/0    00:00:00 tmux attach

Then I immediately noticed that the removing /tmp/tmux-1000/ things made the problem. And thankfully Tmux provides workaround. From the tmux manpage:

-L socket-name

tmux stores the server socket in a directory under /tmp (or TMPDIR if set); the default socket is named default. This option allows a different socket name to be specified, allowing several independent tmux servers to be run. Unlike -S a full path is not necessary: the sockets are all created in the same directory.

If the socket is accidentally removed, the SIGUSR1 signal may be sent to the tmux server process to recreate it.

Now we can revive our Tmux sessions by sending a signal to recreate sockets:

killall -s SIGUSR1 tmux

More simply, since the number of SIGUSR1 is 10:

killall -10 tmux

Then we can do tmux attach successfully. Yay!