Why didn't i know that?

Untill now. .

Thanks o great community for the Vim Faq, entry 32.1 :D :

32.1. I am running Vim in a xterm. When I press the CTRL-S key, Vim freezes. What should I do now?

Many terminal emulators and real terminal drivers use the CTRL-S key to stop the data from arriving so that you can stop a fast scrolling display to look at it (also allowed older terminals to slow down the computer so that it did not get buffer overflows). You can start the output again by pressing the CTRL-Q key.

When you press the CTRL-S key, the terminal driver will stop sending the output data. As a result of this, it will look like Vim is hung. If you press the CTRL-Q key, then everything will be back to normal.

You can turn off the terminal driver flow control using the 'stty' command:

    $ stty -ixon -ixoff