If your account stops working and you can't log in any more, you can contact your server admin's public email address to ask them for help.

Every Mastodon server has a public email address on its About page:

1. Go to your server's website
2. Click "Learn more" (or ⋯ if you're using a phone)
3. The contact email address will be in the middle of the screen labelled "CONTACT:"
4. Send the admin an email explaining your problem

You don't need to be logged in to see a server's About page.