Line 17 of this code reports an error.

using System;
using System.Data;
using System.Configuration;
using System.Collections;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Web.UI.HtmlControls;

public partial class BasePage : System.Web.UI.Page
{
    string langvalue = "sName";
    MySqlClient.Adapter cmd;
    public string SessionID;
    ThinkingSession.ThinkingSession s1 = new ThinkingSession.ThinkingSession();
    public string Language;

I haven"t touched it before. I don"t know why it suddenly won"t work. I just return 500.

error says that the namespace is not found, and you don"t know where the class is placed in the project folder. The problem is that it was normal before.
I don"t know why I suddenly failed. I asked for help.