You got lots of answers about proactive approaches to the issue. In cases where you didn't do those, you'd contact support. If it's a bank account, they ask you

You got lots of answers about proactive approaches to the issue. In cases where you didn't do those, you'd contact support. If it's a bank account, they ask you security questions and then reset/remove MFA. I've done that over the phone while out of the country once. If they don't have support, or their support doesn't have an alternate authN approach, you could be locked out.