I have a few questions to help steer you in the right direction.
1. Did you purchase the domain from Shopify?
2. Is your Shopify account active?
3. If yes ^ - Can you go to Settings/Billing and see if you can see a failed invoice?
Let me know what you find out.
That might be why! If the credit card was expired, the renewal likely failed and would require that we manually renew the domain. I have sent you an email titled "Shopify Forum Support" to verify the store so I can take a look. Please respond to that email with the requested information so we can work together to find a solution.
You will need to reply to the email I sent you and not this forum thread in order to verify your store. If you did not receive the email, I can have it resent.