If your account stops working and you can't log in any more, you can use your 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.