To make Sql Server to be accessed remotely, check the belows.
1. Use correct server name. The server name is machine name + instance name. e.g. My-PC\SQLExpress
The server name shouldn’t start with ‘\\’
2. Tick Allow remote connection to this server from the tab Connection of Server Property from SQL Server Management Studio.
3. Enable TCP/IP Protocol for the SQL server in SQL Server Configuration Mananger.
4. Check the Port number being used from the TCP/IP property from the above. and Add Rule to allow income access of TCP Port 1433 (if this is being used) and UDP 1434.
5. Make sure SQL Server Browser is running. If it’s not running, check if it is disabled.
References
http://blogs.msdn.com/b/walzenbach/archive/2010/04/14/how-to-enable-remote-connections-in-sql-server-2008.aspx
http://blogs.msdn.com/b/sql_protocols/archive/2007/05/13/sql-network-interfaces-error-26-error-locating-server-instance-specified.aspx