Double tap your Home button and see if Mail is open. Swipe left or right to locate it, and if you find it swipe up on its app preview. Next or if you didn’t find the Mail app preview to close perform a force restart on your device by pressing Home and Power at the same time. Maintain this hold until you see the Apple logo.
Check out this article for more details (https://appletoolbox.com/2016/06/mail-not-opening-on-iphone-or-ipad-how-to/)