How to add old database in new SQL server properly

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C#

SQL-Server-2012

I uninstall my old SQLserver and Install new sql server 2012 Now i add My old Database in my sql server 2012 and then i Connect this old database in my C# project An Error is occurred.
Infect I make a new database and make new C# project and then connect it it work good. but my not work with my OLD Database
Please help me how i remove this error.
This Error is occurred

System.Data.SqlClient.SqlException: '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 Interfaces, error: 26 - Error Locating Server/Instance Specified)'

What I have tried:

its my connection string class.

namespace CNG
{
   public class dbconfig
    {
        public static string connectDb()
        {
            string conString = @" Data Source = CTRLSOFT - ELDHL3\SQLEXPRESS; Initial Catalog = CNG; Integrated Security = True";
            return conString;
        }
    }
}