Minecraft app doesn't work

The app opens, shows me the Mojang with white background, asks me for my iTunes account password and then closes the app. I tried restarting my phone but that hasn't worked. It also didn't let me write a review on the App Store(I hadn't heard of this website) as it tells me I cannot write a review without purchasing the app. I have had Minecraft on my phone for over a year and a half, and have never had this problem.