Text no longer shows after killing program, using tmux

Your description indicates that the tty was left in a non-echoing mode after you stopped the server.
This often happens when an “interactive” program exits unexpectedly and does not have a chance to restore the various tty modes it adjusted when it started. If it happens regularly when exiting the program normally, then there is a bug in the program.

You can probably reproduce the effect with a command like this:

stty -echo ; printf 'Type something (there will be no echo); Enter/Control-C to resume...' ; read ; echo

You should be able to use this command (typed “blindly”; since there is no echo) to restore normal tty functionality:

stty sane