Please use anyone of the following connection string for connecting to MS SQL Server 2012 database:-
Connection to a SQL Server instance
------------------------------------
<add key="ConnectionString" value="Data Source=.\MSSQLSERVER2012;Initial Catalog=database_name;User ID=db_user_name;Password=db_password;Persist Security Info=True;" />
Connect to SQL server via TCP/IP Protocol
------------------------------------------
<add key="ConnectionString" value="Data Source=localhost [OR REPLACE_WITH_YOUR_DOMAIN_NAME],1437;Network Library=DBMSSOCN;Initial Catalog=database_name;User ID=db_user_name;Password=db_password;Persist Security Info=True;" />