Start with initial fault finding:
What happens when you try to receive or send email? Do you see any errors - and if so, what do you see?
If email is failing to send or receive, you are likely to see indicative errors at the bottom of the mail sidebar - the wording of the error usually providing sufficient information to determine the nature of the problem.
Deleting and re-adding your email account is sometimes the easiest way to resolve a problem with a specific Mail account. After deleting the account, perform a forced restart before re-adding the email account to your iPad.
Settings > Mail > Accounts > [Your Mail Account] > Delete Account
Force restart iPad - Apple Support
Settings > Mail > Add Account
Be aware that some email providers now require the use of App Specific Passwords when accessing mail over IMAP or POP3. If this applies to your email account, you will need to generate a unique password for your iPad email account - that will differ from your normal Account Password. As examples - AOL, Yahoo and Microsoft all use App Specific Passwords when using anything other than their own email client Apps.
If you need to generate an App Specific Password, you’ll need to log-in to your Mail Providers account management webpage - using a web browser - and generate the Password from the appropriate account security page.
More information about AOL’s App Specific Passwords can be found here:
https://help.aol.com/articles/allow-apps-that-use-less-secure-sign-in?guccounter=1
For Yahoo, more information can be found here:
https://help.yahoo.com/kb/SLN27791.html
Microsoft:
https://support.microsoft.com/en-us/account-billing/using-app-passwords-with-apps-that-don-t-support-two-step-verification-5896ed9b-4263-e681-128a-a6f2979a7944
Google Mail (Gmail) has until recently also used App Specific passwords to access email - although this has now been deprecated in favour of OAuth authentication. Simply removing and re-adding the Gmail account from the Add Account wizard, selecting Google and following the prompts, will correctly configure the correct account settings.
