Re: why won't my email start when I select the mail icon?
I don't know why, but you can try any of the following:
1. Reboot your iPad Air.
2. Double-click on the home button to bring up the list of your most recently used apps, and then swipe up on the Mail app. Afterwards, try to start the Mail app.
3. If the mail app you're using is a 3rd-party app, delete it, and then re-install it.