Terminal got hanged when editing something in vi

Ctrl+Q will undo Ctrl+S. These are ancient control codes to stop and resume output to a terminal. They can still be useful, for instance when you are tailf-ing a log file and something interesting scrolls by, but this era of unlimited scrollback buffers has really obsoleted them.

I would like to complement zwets' accepted answer.

You can see the meaning of special keypresses by issuing the commands stty -a and man stty.

stty -a prints all current settings of the terminal. The result in my terminal:

speed 38400 baud; rows 33; columns 80; line = 0;
intr = ^C; quit = ^\; erase = ^?; kill = ^U; eof = ^D; eol = M-^?; eol2 = M-^?;
swtch = M-^?; start = ^Q; stop = ^S; susp = ^Z; rprnt = ^R; werase = ^W;
lnext = ^V; flush = ^O; min = 1; time = 0;
-parenb -parodd cs8 hupcl -cstopb cread -clocal -crtscts
-ignbrk brkint -ignpar -parmrk > -inpck -istrip -inlcr -igncr icrnl ixon -ixoff
-iuclc ixany imaxbel iutf8
opost -olcuc -ocrnl onlcr -onocr -onlret -ofill -ofdel nl0 cr0 tab0 bs0 vt0 ff0
isig icanon iexten echo echoe echok -echonl -noflsh -xcase -tostop -echoprt
echoctl echoke

man stty prints the manual of stty. The part that is relevant here:

Special characters:

   start CHAR
          CHAR will restart the output after stopping it

   stop CHAR
          CHAR will stop the output