Pressing "Ctrl + S" by mistake while using Vim

Well, It's no a vim issue. This is called Software flow control. There is a bunch of info out there on the internet.

According to an article in here:

CTRL-S (XOFF) is part of Software flow control. XOFF notifies the process or device sending data that the input buffer is full and it shouldn't send any more data. The software (outputting process, firmware, terminal etc) can choose how to handle the notification - i.e. keep running and buffer data, drop data, pause until XON is received, etc.

More info about Software flow control here.

NOTE: if you want to get the control of your terminal after you pressed CTRL + s key combination, you can press CTRL + q.

I hope this helps.