Could Not Open A Connection To SQL Server

Hi,
 
Has somebody had a problem like this before?
Exception Details: System.Data.SqlClient.SqlException:
An error has occurred while establishing a connection to the server.
 When connecting to SQL Server 2005, this failure may be caused by the
fact that under the default settings SQL Server does not allow remote
connections. (provider: Named Pipes Provider, error: 40 - Could not
open a connection to SQL Server)
That's part of the code:
        SqlConnection con;
        string sql;
        SqlCommand cmd;
      
        con=new SqlConnection("Data Source=elf/SQLEXPRESS;Initial Catalog=logos;Integrated Security=SSPI;");
        sql="SELECT max(id_kandydat) from Kandydat";
        con.Open();
        cmd=new SqlCommand(sql,con);
        int id = Convert.ToInt32(cmd.ExecuteScalar());

        con.Close();

        SqlDataSource7.InsertCommand="INSERT INTO Kandydat_na_Kierunek(id_kandydat, id_kierunku, id_stan) VALUES ("+"'"+id+"'"+","+"'"+DropDownList1.SelectedValue+"'"+","+'1'+");";
 
I don't really get it. I'm nearly 100% sure that before some changes in my project this had worked perfectly. Does anybody have any idea how to make it work again? I would be very grateful for any help. And, yesy, I know it is quite common error, but proposed solution found didn't help.
Regards,
N.