@Usman shabbir Thanks for reaching out to us. We do understand your concern. The error message "The tenant was not found" do come in some scenarios where there has been recent changes in credential and browser/ system caches tries to use the older passwords.
The user name and password incorrect would mostly come in same scenario as well. This is normal for few scenarios (Error in typing, old creds cache), it does not necessarily shows that the account was hacked.
Also while we are at this, it would be a great thing to actually enable the MFA on the account as you did mention learning about it. It would prevent any kind of unauthorized access and would be very difficult to hack it.
Also it would be great, if you keep separate credential for other things like your bank and corp. Single password always have more risk.
If you are an admin, it would really be great to have a look at this best practices from us and then implement this in your organization to prevent identity based attacks :
https://learn.microsoft.com/en-us/azure/security/fundamentals/steps-secure-identity
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