After you register a new domain or change its nameservers, it doesn't appear online instantly everywhere in the world. This delay is called DNS propagation.
DNS (Domain Name System) is essentially the internet's phone book - it translates your domain name into the server address where your website actually lives. When you make a change, that update has to spread out to thousands of DNS servers around the world, each of which stores (caches) the old information for a while to keep things running fast.
In practice, most changes are visible within a couple of hours, but it can take up to 24-48 hours to be fully visible everywhere. During this window some people may see your new website and others may still see the old one, or nothing at all - this is completely normal and resolves itself.
If it's been more than 48 hours and things still aren't working, double-check that your domain's nameservers and DNS records are set correctly.