What happens if my domain name expires?

If your domain name expired, then your site will show a advertisement page from the registrar.

A whois lookup will reveal that the domain expires in the future, but was updated a day after it renewed, and the nameservers are changed. (The nameservers will automatically revert back when the registration fee is paid.)

You have 29 days to pay for your domain name and keep it. After 29 days, it will cost extra to renew, and you risk losing the domain to another buyer.