If you do not have auto-renewal enabled for your domain, then you need to manually renew your domain to prevent it from expiring.
If your domain subscription has expired, then you have 40 days to contact Shopify support if you want to renew it. You cannot renew an expired domain yourself. You can learn more about renewing your domain and ensuring your domain is set to auto renew here.
To contact our support directly please follow these steps:
Hope this helps!