Hi there and welcome
Can you hold the home and power buttons on the iPad for about 10 seconds until you see the apple logo then release.
Let the iPad restart and when it has, tell me if you still have the issue please.
Can you please go to settings, Mail, and press on your mail account, then press Delete Account
Once done, repeat the procedure we just did to restart, go to Settings, Mail, add the account back again and tell me how it now functions please