Source: https://smallstep.com/blog/ssh-tricks-and-tips/#exit-stuck-sessions
SSH sessions can often hang due to network interruptions, a program that gets out of control, or one of those terminal escape sequences that lock keyboard input.
ssh includes the escape character ~ by default. The command ~. closes an open connection and brings you back to the terminal. (You can only enter escape sequences on a new line.) ~? lists all of the commands you can use during a session. On international keyboards, you may need to press the ~ key twice to send the ~ character.