Can any one please help me in that error? Thanks

When I right click in solution explorer and add new item SQL Server Database I got this error:

A network-related or instance specific error occurred while establishing a connection to SQL-Server. The server was not found or it was not accessable. Verify that instance name is correct and SQL Server is configured to allow remote connections. (Provider: SQL network interfaces, error: 26 - Error locating Server // instance specified)