In this case, mostly, it is caused by network issue.The quick solution for this, is to resetting your device.
Look the above picture and follow the steps below:
- Press and hold down the sleep/wake button at the top of you iPhone or iPad.
- At the same time, press and hold down the Home button on the front of your iPhone or iPad.
- Keep holding both buttons until the screen turns off and then turns back on again and displays the Apple logo.