can neither get or receive mail on Mail on iMac

You have given no information to evaluate and respond to, so I will offer the following suggestions.

• Confirm that you have access to the internet. Are you able to use a browser?
• Something happened to remove or corrupt your ID or passwords. You can open Preferences for the email app and carefully check what is shown. Correct as necessary.
• If access still does not work, use your browser to access the email server. Does the password and ID work?
• If so, then delete the account from your email app , create a new one and reenter the ID and passwords. That generally resolves the problem.