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.
@feditips what if the whole server is just down? (Happened to me before)
If it's down for everyone, there's probably not much point contacting the admin. They should be aware of the issue.
However, if you think they might not be aware, and if you haven't written their email address down previously, you can find a copy of your server's website on the Wayback Machine at https://archive.org. This should include the About page and its email address.