Below is a very common problem that could occur after a new install of SQL Server on a server. When you try to access it over the network, the following error pops up in one of the below forms:

A network related or instance specific error occurred, while establishing a connection to SQL Server.

  

The server was not found or was not accessible. Verify that the instance name is correct and that SQL server is configured to allow remote connections.(provider: SQL Network Inrerfaces, error:26 -

 

Error Locating Server/Insatance Specified)(Microsoft SQL Server)

Follow the procedure in the link below in SQL 2008 R2:  
http://www.byteblocks.com/post/2009/04/02/Configure-SQL-Server-to-allow-remote-connections.aspx