I changed my email domain to a new domain and now I cannot log in through a single sign-on login method.
Example: Old domain email@example.com was changed to firstname.lastname@example.org
Even with SSO enabled, you can still access your account using your native Zendesk authentication credentials. Use this backdoor link, https://mycompany.zendesk.com/access/normal, to access your account using native Zendesk authentication. Replace "mycompany" in the example link with your own Zendesk subdomain.
Once you log in, make any necessary changes to SSO settings as well as user email addresses. If you are using both domains during a transition period then both domains should exist on your user profiles.
For more information about what to do when your SSO service is down, see the article: Accessing your Zendesk account when your SSO service is down