Firstly, email is usually a “paid” service that is included with a service subscription. If you cancel your contract, you will likely lose access to the associated email platform - unless you have the option to pay to receive ongoing email service. On the basis of the information included in your post, this may be the root-cause of your issue.
Otherwise, has your email provider switched to using App Specific Passwords? If so, you will need to generate a unique password for your iPad email account - that will differ from your normal Mail Account Password. As examples - AOL, Yahoo, Google, Microsoft all use App Specific Passwords when using anything other than their own email client Apps. Many commercial operators partner with one of these (or other) operators to provide email service.
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
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