You may need to add a name server for your domain if you've accidentally deleted one of your name servers. Also, some high availability sites may need more than 2 name servers declared.

Step by Step Instructions:

1. Log in to your domain profile (see this page for instructions)
2. On the menu bar, select Services->DNS Services->DNS used by this domain.
3. In the space provided next to Add Name Server, type in the name server you need to add.
4. Click on Save Configuration
5. Keep in mind, you should have a minimum of 2 name servers for your domain, and these should be from the same service supplier.