Hank here from Shopify.
Are you able to tell me a bit more about this as domain issues can be quite complex?
Did you recently change any DNS settings with your domain? That is the most common cause for any domain-related issue as it can take up to 48 hours for DNS propagation to properly connect.
Or have you just bought, or renewed the domain? How long before the issue itself happened, or do you remember the last time it was working correctly for you?
If you can share the exact error message, and also your domain URL itself, and I can take a closer look at this.
All the best, Hank