I've just started using tmux recently, and have run into this problem a few times.
Whatever I type on the command line, is hidden from view until I press enter and it is executed. I don't know how I triggered this and can't find any mention of 'hidden input' in the manual. I've tried setw -g monitor-activity on, and set -g visual-activity on , but it seems these are meant for something else.
To be clear, I can run a program or command if it's typed correctly -- e.g. python manage.py runserver works, but while I am typing it -- no keys show up. It's the same as when you are entering a password for PostgreSQL or similar. The commands do show up in history and can be seen after the command is executed (above the current line).