I've resolved the issue. It was due to the SQL browser service.
Solution to such problem is one among below -
-
Check the spelling of the SQL Server instance name that is specified in the connection string.
-
Use the SQL Server Surface Area Configuration tool to enable SQL
Server to accept remote connections over the TCP or named pipes
protocols. For more information about the SQL Server Surface Area
Configuration Tool, see Surface Area Configuration for Services and
Connections.
-
Make sure that you have configured the firewall on the server
instance of SQL Server to open ports for SQL Server and the SQL Server
Browser port (UDP 1434).
-
Make sure that the SQL Server Browser service is started on the server.
link - http://www.microsoft.com/products/ee/transform.aspx?ProdName=Microsoft+SQL+Server&EvtSrc=MSSQLServer&EvtID=-1