First off an email provider will never permanently delete an account, so the email was likely a phishing email trying to get you to go to to some site or download something to get a foothold on your system or get your credentials. If you went to any provided links and entered your credentials you will want to reset your password ASAP. If you did enter creds someone probably change your email password so they could use the account for spam or other phishing attacks and that is why you would get a popup since your password is no longer valid.