Not a normal situation, but an application can get confused and hang… which results in nothing happening when you tap the screen or tap the Home button. Before you restart the whole system as we mentioned before here, you can just force quit the one application.
Force quit an application is as simple as… holding the Home button down for more than six seconds. This will shut down the application and take you back to the launch screens of icons. Of course, if this doesn’t work your back to a whole system restart. Give this a try first.