Worked offline with Matt to understand the issue.
Issue details: Due to change in AD Connect Server configuration, domain was changed from federated to managed and also none of the onmicrosoft.com admin accounts exist in the tenant due to which none of the users/Global admin accounts were able to access emails/portal for admin tasks.
Collaborated with Data Protection team and after verification, we were able to recover the Global admin account access, created a onmicrosoft.com account as Global admin.
Still existing users were unable to login, followed the steps from this article https://learn.microsoft.com/en-us/microsoft-365/enterprise/turn-off-directory-synchronization?view=o365-worldwide disabled the dirsync and after which able to reset the password of the existing user accounts and verified they were able to access emails.
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