problem; obtain value in detailsview cells

Can  you tell me why I  can not access the value of a textbox in the detailsview control?

I am working on a new C# asp.net 2010 webforms  application. I am trying to obtain the value loaded into a textbox(cell) for a detailsview control. When I step through the code, the debugger says the value is NULL.  However when  I run new the web page, there is the correct value loaded into the cell(textbox).

When I am trying to access the cell, I am clicking on a button that is not inside of the detailsview control. This is the requestor's preference. If I need to use only the buttons in the detailsview control, please let me know and how would you code it?

The following is the code I have right now:

<%@ Page Language="C#" MasterPageFile="~/RpApp.Master" AutoEventWireup="true"
     CodeBehind="Detail.aspx.cs" Inherits="Support.Att_Detail" %>

             Select="new (Att_id, Prior_Cutoff_Date)"
         TableName="Attestations" Where="Att_id == @Attid">
        
            
        
    
  
             AutoGenerateRows="False" DataSourceID="LinqDataSource1" BorderStyle="Solid" HorizontalAlign="Center">
        
            
                
                                              BorderStyle="None">
                
            
                      
           
               
               
                    
                
                 
                    
                  
               
               
           
        
    

                                             ID="btnNext" 
                                       Text="NEXT"
                                       runat="server"
                                       onClick="NextButton"
                                        CssClass="RptFormButton" /> 

--------

     The following is the code in the aspx.cs file

protected void NextButton(object sender, EventArgs e)
         {
               
             

                TextBox tb = (TextBox)DetailsView1.FindControl("TextBox1.Text");
                 string name = DetailsView1.Rows[0].Cells[1];

}