Frozen screen

Something else you can try if the phone screen itself is responsive is to completely close the mail app, restart your phone and then launch the mail app again.

Close an app on your iPhone or iPod touch - Apple Support

If the screen is totally unresponsive, then Katana-San’s suggestion is the way to go. I added to this the discussion because an app can freeze, but the screen can still be responsive.

Hello ~ Any help here?

If your iPhone won't turn on or is frozen - Apple Support

~Katana-San~

Hello ~ Great addition and always a pleasure to be in a discussion where you are present.

~Katana-San~

Likewise. 🙂

Hello ~ Glad you are back to normal.

~Katana-San~