When you run into this error, it may mean emails are not being sent correctly from your WordPress site out to your community. Most of the time the culprit can be detected in the configurations mentioned here.
Follow these steps to troubleshoot:
1.) Confirm that the login credentials used to set up SMTP plugin have not been updated.
2.) Confirm your SMTP server address, TLS Port number, or SSL Port number with your email service provider to double check you are using the correct SMTP settings
3.) If you recently added or activated a 2-factor authentication for your email, the SMTP connection cannot be established. In this case you either need to turn it off or generate an app password for your email account
4.) If all is well at this point, get in touch with your email provider, or the person who configured your account, to ask if the SMTP authorization has been disabled. If that is the case, you will need to enable it again.