Do as madiba says about turning it off in settings, re the battery life, let the phone run down completely, then give it a good charge, that usually helps. Do you have any apps that maybe running, double click the start button, the screen will rise and show you all the apps that are running, hold finger on one until they all shake, then press the red circle, do it for all of them. This closes everything, just open what you need as and when you need it.