Thank you for posting this in Microsoft Q&A.
This MFA registration prompt is coming up due to security defaults.
If your tenant was created on or after October 22, 2019, security defaults might be enabled in your tenant. To protect all of our users, security defaults are being rolled out to all new tenants at creation.
To help protect organizations, we're always working to improve the security of Microsoft account services. As part of this protection, customers are periodically notified for the automatic enablement of the security defaults if they:
- Haven't enabled Conditional Access policies
- Don't have premium licenses
- Aren’t actively using legacy authentication clients
After this setting is enabled, all users in the organization will need to register for multifactor authentication. To avoid confusion, refer to the email you received and alternatively you can disable security defaults after it's enabled. All users have 14 days to register using the Microsoft Authenticator app or any app supporting OATH TOTP. After the 14 days have passed, the user can't sign in until registration is completed. A user's 14-day period begins after their first successful interactive sign-in after enabling security defaults.
As per your current situation looks like you are unable to sign-in because 14days have already passed.
In this situation if you are the only global admin on the account and are blocked entirely, you can reach out to our support team. You can look into below article to get support numbers depending on your country.
https://support.microsoft.com/en-us/topic/global-customer-service-phone-numbers-c0389ade-5640-e588-8b0e-28de8afeb3f2 or creating a ticket through a different account: https://learn.microsoft.com/en-us/microsoft-365/admin/get-help-support?view=o365-worldwide#phone-support Create a ticket with Microsoft support team. Give them the tenant ID which is locked out in your description. Tell them that no admin account has access anymore and your partners also have no access anymore. Once you create a ticket with support team you will have to work with our data protection team. You will have to first prove your identity against your tenant for security purpose. Post that this team will help you with help you in getting access to your tenant or unlock your account depending on your scenario. Also, for the future, you can create an emergency access account (break glass) in Azure AD. This account will help prevent being accidentally locked out of your Azure Active Directory (Azure AD) organization because you can't sign in for any reason. https://docs.microsoft.com/en-us/azure/active-directory/roles/security-emergency-access Let me know if you have any further questions.
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