Press and hold down the 'Home' and 'Power' buttons at the same time for upto 10 seconds. Then you close the app that has this issue. You just need to press the recent applications menu (usually the first left button) in your phone.
Most of the times, it might be a temporary loading issue. Usually when you open an app, you will see a black screen for few seconds and then app will crash with or without an error message. It is one of the most common problem in android operating system. Shadow of Chernobyl App Problems and Troubleshooting Steps I'm getting black screen (blank screen) when I open S.T.A.L.K.E.R.
