After a period of inactivity, the screen turns off to conserve battery power. 
	 
You can set the idle time before the screen turns off. 
  
 
	 -  
		  Go to Settings, and then tap 
				Display & gestures or 
				Display, gestures & buttons. 
			  
		
 
-  
		  Tap 
			 Screen timeout, and then tap the time before the screen turns off.