Hi, I'm Ashik. Welcome to JustAnswer. I'm reviewing your question now, and will post back with your reply ASAP
There are several reasons for this problem. It is possible that the iPhone firmware/software is glitched or it is possible that the app itself is corrupted.
First we can reset your iPhone and see if it works or not. Press and hold down the Power button and Home button at the same time until it shows the Apple logo. When you see the Apple logo, release the buttons and let it boot.
Note : Power button is the button you use to turn off your iPhone or its screen. Home button is the round button at front bottom.
Check the app. If that didn't work, then we need to delete that app and redownload it from App Store. Tap and hold down the app and it will start wiggling. Tap the x button on it and it will be deleted. Then open App Store, search that app and download it again.
Please let me know if that didn't work, so we can continue troubleshooting.