Minecraft relaunching on blank screen

My sons both have Minecraft PE on their Amazon Fire HD tablets. Recently, on leaving Minecraft, and then re-launching, Minecraft does not launch, only a blank screen. The device then has to be rebooted for Minecraft to work again. I have uninstalled the app, and restarted, to ge the same issue. Is this a bug? Please help, I have 2 very upset boys, who just want to play!