Server Not Found Error.

Here are the main reasons:
1. Please check the domain whether it has registered. You can done by checking the who is information.

2. If you you have changed the name server or DNS record then your domain may not be visible for 24-48 hours, as it takes time for the new information to propagate to all of the DNS servers around the world.
You can use the following URL to check Whois information and DNS record.
http://www.dnsgoodies.com/