Welcome to iMore! Try swiping the app store out of multitasking. Click the home button twice, then find the app store and swipe it out. Then do a hard reset.
7 Reset
Hold the power/wake button and the volume down button at the same time until the Apple logo appears. Then release.
Older phones:
Hold the top or side power/sleep button and the home button at the same time until the Apple logo appears, then release.