C-b C-z suspends your tmux client according to the manpage. I was curious what it did and tried the key combination. It suspended tmux for sure but didn’t put me in the bash shell so I was stuck in limbo and couldn’t issue an fg like I was hoping to.
The way to get back is to open a new session (SSH in on a new tab), find the PID of the tmux process (ps -a | grep tmux does the trick) and then do kill -CONT <PID> (where <PID> is the process ID you found).