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Since you didn’t tell us, what exactly you’ve already done to solve this problem, I’ll suggest the usual fixes:
Force close the Mail app (swipe upwards from the bottom to see the App Switcher, then slide the thumbnail towards the top of the screen).
Restart your iPad.
Make sure date and time are correct (Settings - General - Date & Time).
Go to Settings - Passwords & Accounts - Accounts, select the email account/s and disable Mail. Enable it again after a few minutes.
Remove the email account/s from your device and install it/them again.