Sometimes, in vi, you type 【Ctrl+s】 and the screen freeze.
Press 【Ctrl+q】 to unfreeze.
xterm &to launch xterm. (must be xtrem, not other modern terminal app.)
vi ~/.bashrcto open your bash init file in vi.
:qto exit vi. 〔☛ Emergency vi (vi tutorial)〕
These are ancient keys from 1980s or perhaps earlier. They still work in many boot screen. They are used to send a signal to the device to tell it to stop receiving transmission. It'd needed because, back then, devices often can't keep up with incoming data.
To turn it off, put this in your bash init file:
stty ixoff -ixon
〔☛ What's the Difference Between .bashrc, .profile, .bash_profile, …?〕
For detail of other terminal control sequence keys, see: Linux: Terminal Control Sequence Keys (tty, stty) ⌨