Are you seeing any errors when attempting to access or download email? Any errors will be seen at the bottom of the Mail Sidebar on the left of your screen.
The most common recent cause of difficulties accessing email, following a change in either email settings or a system update, is that your email provider has changed their email to require App Specific Passwords.
If this is the problem, 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 Google 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
For Gmail:
https://support.google.com/mail/answer/185833?hl=en-GB
If your email provider is not one of those listed above, you will need to check requirements from your providers technical support pages.