@Robert Balfe Thanks for reaching here! You may want to know that the App Service domain you bought is valid for one year from the time of purchase. You can configure to renew your domain automatically, which will charge your payment method when your domain renews the following year. You can also manually renew your domain name.
If you want to configure automatic renewal, or if you want to manually renew your domain, follow the steps here- https://learn.microsoft.com/en-us/azure/app-service/manage-custom-dns-buy-domain#renew-the-domain
If auto-renew option wasn't enabled for the domain and it got expired, then please let us know.