Renew it now
- Log in to your registrar. If you are not sure who that is, the free check tells you.
- Find the domain and renew it. Most registrars offer several years at once, and multi-year renewal is the single most effective fix here.
- Confirm the new expiry date shows in the account afterwards. Do not assume the payment went through.
Then fix why you nearly missed it
- Check the card on file. Auto-renew failing on an expired card is the most common cause of a lapse, and the failure notice goes to the same address as everything else.
- Move the contact address. If the domain is registered to a personal address, or to an address at the very domain that is expiring, change it to something that survives an individual leaving.
- Add a second contact. One unread inbox should not be the only line of defence.
- Turn on transfer lock while you are in there. It costs nothing and stops the domain being moved without an extra confirmation step.
If it has already expired
The registry stops publishing the records the moment it expires, so the site and the email both stop immediately. Confusingly, the registrar panel may still show the domain as active, because renewal is technically still possible. To everyone else on the internet it is already gone.
Most registries allow a grace period at the normal price, then a redemption period at a much higher fee, then release. The lengths vary a great deal by extension, so check the page for yours: .com, .co.uk, .io, .de and others are listed under guides.
Act on the same day. Once the domain is released it can be registered by anyone, and expired domains with any history are picked up automatically within hours.
What to write down afterwards
The renewal date, which registrar holds it, and which account it sits on. Put that somewhere other than the registrar's own email, because that is the thing that failed.
On why the notice did not reach you: your registrar's renewal email is not a safety net.