I do assume your question is about what to do in case Azure Entra ID or an on-premise Active Directory is not available. In that case, there is no option to log in as the authentication is only managed using Entra ID/Active Directory.
Microsoft guarantees an uptime of 99,9% of their Azure services. In case your environment is using an on-premise authentication method, you need to ensure that you have a team and processes to keep this available as well.
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