Read the row that matches your situation. The proof each company asks for is different, and asking for the wrong thing wastes a week.
Easiest
You lost the CMS login only
How to check: The site is up and fine. You just cannot log in to edit it. You still have hosting access, or your host still recognizes you as the account holder.
The fix: Use the password reset if the admin email still works. If it does not, hosting access outranks the CMS: from the hosting control panel you can reset a WordPress password directly in the database, or create a new admin user. This is a same-day fix and it costs nothing.
Common
You lost the hosting account
How to check: You know the host but not the login, or the account was opened in a developer name. The site still works, because hosting keeps running while the bill is paid.
The fix: Call the host, do not email. They will want proof you are the account holder: the account number or username, the invoice or billing email, the last four digits of the card on file, or a security PIN set on the account. If the account is genuinely in someone else name, the host will not hand it over, and the real route is to move the site rather than fight for the account.
Common
You lost the DNS but still have the domain
How to check: The nameservers point at a service you cannot log in to. The site works until something needs changing, and then nothing you do has any effect.
The fix: This one is straightforward, because control of the domain overrides DNS. From the registrar you can point the nameservers somewhere new. Copy every existing record first, including MX records, or you will take email down with the website.
The hard one
The domain is in someone else account
How to check: The ICANN lookup names a registrar you never signed up with, or the account is in a developer or agency name. Everything else depends on this, so it comes first.
The fix: Ask, in writing, for the domain to be pushed to an account in your name, or for the authorization code so you can transfer it to your own registrar. If that fails, contact the registrar directly and open a dispute. What decides it is who is recorded as the registrant and who can document paying for it. Keep invoices, emails, and payment records: they are the case.
Urgent
The domain already expired
How to check: The site and the email both stopped at once, on the same day, with no warning. That combination is nearly always domain expiry rather than a hack or a host failure.
The fix: Get to the registrar today. During the auto-renew grace period, paying the renewal fixes it. After that, in redemption, it can still be recovered but there is a restore fee that is much larger than the renewal. After redemption ends the name can be registered by anyone, and there is no route back.
Varies
You have the domain but not the site files
How to check: You control the domain and DNS, but nobody can get into the server that holds the site. Nothing you own contains the pages.
The fix: The domain is enough to rebuild. Pull whatever pages survive from search engine caches and the Internet Archive, rebuild on hosting you control, then point DNS at it. It is not free, but it is a fixed amount of work with an end date, which a hosting dispute is not.