I have 9.3.5 OS and since 2 days am unable to open my mail. I am clicking on the mail icon, which I see is increasing in number of emails (I can see the numbers increasing) but it is not opening…it tries and then brings me back to the home screen.
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/)