Question

it does not gave any value

using (SqlCommand cmd = new SqlCommand("SELECT SUM(Paied) FROM Debt", new SqlConnection(Program.ConnectionString)))
            {
                cmd.Connection.Open();
                SqlDataReader myReader = cmd.ExecuteReader();
                while (myReader.Read())
                {
                    TotalPaiedAll = Convert.ToDecimal( myReader["Paied"].ToString());
                }
                cmd.Connection.Close();
            }

Answer 1

The reason you’re not getting a value is because SUM(Paied) generates an anonymous column, not a column named Paied.

Here is a simple fix for your problem:

        using (SqlCommand cmd = new SqlCommand("SELECT SUM(Paied) As SumOfPaid FROM Debt", new SqlConnection(Program.ConnectionString))) 
        { 
            cmd.Connection.Open(); 
            SqlDataReader myReader = cmd.ExecuteReader(); 
            while (myReader.Read()) 
            {
                TotalPaiedAll = Convert.ToDecimal(myReader["SumOfPaid"].ToString()); 
            } 
            cmd.Connection.Close(); 
        }

And here is a better approach, using the ExecuteScalar method:

        using (SqlCommand cmd = new SqlCommand("SELECT SUM(Paied) As SumOfPaid FROM Debt", new SqlConnection(Program.ConnectionString)))
        {
            cmd.Connection.Open();
            TotalPaiedAll = (decimal)cmd.ExecuteScalar();
            cmd.Connection.Close();
        }