After pressing the app button on my phone's home screen the screen goes black. there is a noticeable portrait to landscape motion just before this happens. 

The screen stays black and doesn't respond to touch and the nothing else happens. Pressing 'back' does nothing, pressing menu button shows up all open apps so you can swipe Minecraft closed from here. Re-opening has no effect on the above.

I have restarted the phone with no change to the above.