Thank you for reaching out to the community forum!
I’m sorry to hear that you are unable to access B2C tenant as the exclusive administrator had issue with the MFA access device.
If you have one account with admin role and if you are blocked entirely, you can reach out to our support team. please refer the below article to get support numbers depending on your country.
Global Customer Service phone numbers
or creating a ticket through a different account with the tenant ID by inform them stating as that you are unable to access the B2C tenant due to loss of MFA device.
Once the ticket is created with support team you will have to work with our data protection team and validate your identity against your tenant for security purpose, post which this team will help you on your issue.
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.
Hope this helps. Do let us know if you any further queries.
Reference: Microsoft support team
Manage emergency access accounts in Microsoft Entra ID
Thanks,
Akhilesh.
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