[Solved] GridView TextBox looses data and shows Empty on PostBack of Button Click in ASP.Net

I add this code but textbox value show in empty in database ..please give me solution.

On Button Click the GridView TextBox loose all data.

public partial class keyexam2 : System.Web.UI.Page
{
    
    //SqlConnection con = new SqlConnection(@"Data Source=.\SQLEXPRESS;AttachDbFilename=D:\shubhangi\jayonline\paper\App_Data\QUIZ.MDF;Integrated Security=True;User Instance=True");
    DataTable dt = new DataTable();
    //DataRow row = dt.NewRow();
    SqlCommand cmd;
    SqlDataAdapter sda;
    SqlConnection con = new SqlConnection(ConfigurationManager.ConnectionStrings["ConnectionString"].ToString());
    protected void Page_Load(object sender, EventArgs e)
    {       
       
        //String strConnString = System.Configuration.ConfigurationManager.ConnectionStrings["Data Source=.\SQLEXPRESS;AttachDbFilename=D:\shubhangi\jayonline\paper\App_Data\QUIZ.MDF;Integrated Security=True;User Instance=True"].ConnectionString;
        string strQuery = "select * from OneQuestion order by QuestionID";
        cmd = new SqlCommand(strQuery, con);
        //SqlConnection con = new SqlConnection(strConnString);
        SqlDataAdapter sda = new SqlDataAdapter();
        cmd.CommandType = CommandType.Text;
        cmd.Connection = con;
        try
        {
            con.Open();
            sda.SelectCommand = cmd;
            sda.Fill(dt);
            //GridView1.DataSource = dt;
            //GridView1.DataBind();
            //Gridview1.DataBound = dt;
            //Gridview1.DataBound();
            Gridview1.DataSource = dt;
            Gridview1.DataBind();

        }
        catch (Exception ex)
        {
            Response.Write(ex.Message);
        }
        finally
        {
            con.Close();
            sda.Dispose();
            //con.Dispose();
        }
        Gridview1.DataBind();
  
    }

    protected void Gridview1_SelectedIndexChanged(object sender, EventArgs e)
    {

    }

    protected void Button1_Click(object sender, EventArgs e)
    { 
         
   
        con.Open();

        foreach (GridViewRow gr in Gridview1.Rows)
        { 
            string data1 = gr.Cells[0].Text;
            string data2 = gr.Cells[1].Text;
            
            TextBox box1 = (TextBox)gr.Cells[2].FindControl("textbox1");

        

                
            cmd = new SqlCommand("insert into Answertbl(QuestionID, Title,Answer1) values ('" + data1.ToString() + "', '" + data2.ToString() + "', '" + box1.Text + "')", con);
                    
            cmd.ExecuteNonQuery();
                
            

        }
        con.Close();
        lblshow.Text = "success";
    }
}