ASP.Net Error: Input string was not in a correct format

Hello Sir,

Getting error in below code:  Input string was not in a correct format.

 SqlConnection con12 = new SqlConnection(ConfigurationManager.ConnectionStrings["abc"].ConnectionString);
        String strQuery = "select Id,care_level  from table1 where" +
                          " Id = @Id";

        SqlCommand cmd12 = new SqlCommand();
        cmd12.Parameters.AddWithValue("@Id", DropDownList1.SelectedItem.Value);
        cmd12.CommandType = CommandType.Text;
        cmd12.CommandText = strQuery;
        cmd12.Connection = con12;
        try
        {
            con12.Open();
            SqlDataReader sdr12 = cmd12.ExecuteReader();
            while (sdr12.Read())
            {
                txt_value.Text = sdr12[1].ToString();
                hcp.Visible = true;
            }

            string query = "SELECT top 1 [Level 1],[Level 2],[Level 3],[Level 4],[Level 5] from table2 order by Id desc;";
            SqlCommand cmd = new SqlCommand();
            cmd.CommandType = CommandType.Text;
            cmd.CommandText = query;
            cmd.Connection = con;

            try
            {
                con.Open();
                SqlDataReader sdr = cmd.ExecuteReader();
                while (sdr.Read())
                {
                    if txt_value.Text == "Level 1")
                    {
                        TextBox2.Text = sdr[0].ToString();
                    }
                    
                     if txt_value.Text == "Level 2")
                    {
                        TextBox2.Text = sdr[1].ToString();
                    }

                     if txt_value.Text == "Level 3")
                    {
                        TextBox2.Text = sdr[2].ToString();
                    }
                     if txt_value.Text == "Level 4")
                    {
                        TextBox2.Text = sdr[3].ToString();
                    }

                     if txt_value.Text == "Level 5")
                    {
                        TextBox2.Text = sdr[4].ToString();
                    }
                }
}
catch(Exception)
{
throw ex;}
}
catch(Exception)
{
throw ex;}

Please help.

Thanks