@ Henry_Best
I chosed to use case because later i will be needing to do different tasks in different cases. Ok, maybe for now it wasn't necessary and Serial.println would be easier.
@ PaulsS
Ok, some comments are useless and can be deleted. The program has purpose, and it works first time but after other button presses it lags and doesn't count down seconds. void keypadEvent does everything for me so i don't need really keypad.getKey(); value.
Explaining again - if button state has changed, program starts do while loop in which seconds counting down and turns on/off led. If correct password is entered do while loop ends.