Sole Admin Locked out of Microsoft tenant -- MFA Error 500121

Hey, sorry you're dealing with this, lockouts like this are honestly one of the most stressful tenant situations out there. The good news is it's recoverable.

Quick context on 500121: it means the MFA step itself is failing, not your password. So usually the registered method is gone or broken somehow, like a wiped phone, the Authenticator app got deleted, or number matching isn't completing.

Before going down the support route, a couple of quick things worth ruling out. On the sign in screen, try "I can't use my Microsoft Authenticator app right now" or "Sign in another way" in case there's a fallback method (SMS, call, backup code) still registered that you've forgotten about. I'd also test it in an incognito window or on a different network, since I've seen 500121 turn out to be a temporary policy or cache issue rather than a real lockout.

If you genuinely are the only Global Admin and have no working second factor, there's no self service path here (it's intentional, for obvious security reasons), so you'll need Microsoft support to reset MFA on the account. Since you can't sign in to raise a ticket, you'll have to call in. The Microsoft 365 billing/subscription line can route you to the right identity team. When you get through, be specific: "I'm the sole Global Admin, locked out by MFA error 500121, and I need MFA reset on my account." That phrasing tends to get you to the correct queue a lot faster.

One thing to have ready: they'll verify that you actually own the tenant, so line up your tenant ID, the domain (you may be asked to add a DNS TXT record to prove ownership), and your billing details. If you happened to purchase through a CSP/partner, reach out to them first, they can often sort this out faster than going direct.

Once you're back in, I'd really recommend setting up a dedicated break glass global admin account and registering a couple of backup MFA methods on your own account too, so a single lost device can never lock you out again. Happy to share how I usually configure the break glass account if that'd help.

Hope you get back in soon, let us know how it goes!