what is the button that blocks your screen?

[AC Question]

there was an app on my phone. I pressed on it and it won't let me press anything on my phone. it has blocked the screen.

 

[B. Diddy]

Welcome to Android Central! Which phone, and which app? You can always force a shutdown by either pulling the battery (if possible) or pressing and holding Power for 10-15 seconds (or Power and Vol Down, if on a Samsung device).