How Do I Solve "Networkerror 500 Internal Server Error - Http //Localhost"

<script type="text/javascript" language="javascript">
   $(document).ready(function() {
$.ajax({
type: "POST",
contentType: "application/json; charset=utf-8",
url: "QuotaMaster.aspx/BindDatatoDropdown",
data: "{}",
dataType: "json",
success: function(data) {
$.each(data.d, function(key, value) {
$('#<%=ddlmode.ClientID%>').append($("<option></option>").val(value.mode_id).html(value.planmode));
});
},
error: function(result) {
alert(result);
}
});
});
    
    </script>

[WebMethod]
protected static BindModeDetails[] BindDatatoDropdown()
{
   try
   {
      DataTable dt = new DataTable();
      List<BindModeDetails> bindMode = new List<BindModeDetails>();
      using (SqlConnection con = new SqlConnection(ConfigurationManager.ConnectionStrings["constring"].ConnectionString))
      {
         using (SqlCommand cmd = new SqlCommand("select mode_id ,planmode From tbl_planmode",con))
         {
            if (con.State == ConnectionState.Closed)
            {
               con.Open();
            }
            SqlDataAdapter da = new SqlDataAdapter(cmd);
            da.Fill(dt);

            foreach (DataRow drRow in dt.Rows)
            {
               BindModeDetails mode = new BindModeDetails();
               mode.mode_id = int.Parse(drRow["mode_id"].ToString());
               mode.Planmode = drRow["planmode"].ToString();
               bindMode.Add(mode);
            }

            if (con.State == ConnectionState.Open)
            {
               con.Close();
            }
         }
      }
      return bindMode.ToArray();
   }
   catch (Exception ex)
   {
      throw ex;
   }
}

public class BindModeDetails
{
   public int mode_id
   {
      get;
      set;
   }
   public string Planmode
   {
      get;
      set;
   }
}