When a domain expires the registry point the domain to a placeholder page while they give you the grace period (usually 15 days) to renew your domain registration. Any service you have in the domain such as email services, etc. will stop working if the domain expires.
Once you renew your domain the registry will point back the domain to the original nameservers, the propagation for this change can take up to 72 hrs so is the time that your site and services can take to work again.
To avoid this happen renew your domain early and before it expires.
