Try this first: tap on the home button twice. The list of recently used apps will open. Find Amazon app, and swipe up to close it. Go to the Home screen, find Amazon icon, and tap on it. Hopefully, it will work just fine.
If the Amazon app is not up to date that may be the reason why you have the problem with it. Go to the App Store, and see if you can update it.
Press and hold the sleep/wake button, and drag the slider to turn off your device. Again, press and hold the sleep/wake button in order to turn your iPhone again.
To do this, find the Amazon app on the Home screen, place your finger on it, and hold. You will see an X – click on it. The app will be deleted. Go to the App Store, find Amazon, download and install it on your device.
Back up your files, and navigate to Settings. Tap on General, Reset, and choose Erase all Content and Settings. You will have to confirm the action, and in order to do so click on OK. When the task is done, set up your iPhone as a new device.