DropDown indexChanged event and button onClick event are clearing my textBoxes

<%@ Page Title="Réparation" Language="C#" MasterPageFile="~/TP/MasterPageTP.master" AutoEventWireup="true" CodeFile="ModifRep.aspx.cs" Inherits="TP_ModifRep" %>
<asp:Content ID="Content2" ContentPlaceHolderID="ContentPlaceHolder1" Runat="Server">
     <div id="content">
    <br />
<br />
<h2>Matériels en réparations</h2>
<asp:Panel ID="Panel1" runat="server" align="center">
<table style="border-color: #000099; height: 918px; width: 688px" border="1">
 <tr>
        <td>
                  <asp:Label ID="Label18" runat="server" Text="Diagnostique du préstatire"></asp:Label>
              </td>
        <td>
                 <asp:TextBox ID="txtSuivi" runat="server" CssClass="TextBox" 
          Height="94px" Width="288px" TextMode="MultiLine"></asp:TextBox>
              </td>
        </tr>
          <tr>
        <td>
                  <asp:Label ID="Label20" runat="server" Text="Statut"></asp:Label>
              </td>
        <td>
                <asp:DropDownList ID="cmbStatut" runat="server" CssClass="Combobox" 
                Height="32px" Width="170px" AutoPostBack="True" AppendDataBoundItems="true"
                onselectedindexchanged="cmbStatut_SelectedIndexChanged" >
            </asp:DropDownList>
              <br />
            <asp:Label ID="lblErrorSta" runat="server" ForeColor="Red" 
                Text="Veuillez saisir le statut" Visible="False"></asp:Label>
              </td>
        </tr>
          <tr>
        <td>
                  <asp:Label ID="Label19" runat="server" Text="Date de récéption  du matériel"></asp:Label>
              </td>
        <td>
               <asp:TextBox ID="txtDateReception" runat="server" CssClass="TextBox" 
                Height="25px" Width="162px"></asp:TextBox>
            <asp:CalendarExtender ID="txtDateReception_CalendarExtender" runat="server" 
                Enabled="True" TargetControlID="txtDateReception">
            </asp:CalendarExtender>
              </td>
        </tr>
          <tr>
        <td>
                 <asp:Label ID="Label21" runat="server" Text="Nouveau service tag"></asp:Label>
              </td>
        <td>
            <asp:TextBox ID="txtNvSt" runat="server" CssClass="TextBox" Height="25px" 
                Width="162px"></asp:TextBox>
              <asp:Label ID="lblST" runat="server" Text="Label"></asp:Label>
              </td>
        </tr>
    </table>
 <asp:Button ID="btnValider" runat="server" CssClass="blueButton"
                Text="Valider" onclick="btnValider_Click" />
 </asp:Panel>

protected void btnValider_Click(object sender, EventArgs e)
    {
         if (cmbStatut.SelectedIndex==2)
                {if (string.IsNullOrEmpty(txtNvSt.Text))
                        {
                            con.charger("update Materiel set reparation= NULL where serviceTag='" + txtServiceTag.Text + "'", false);
                            Session["ST"] = txtNvSt.Text;
                            Response.Redirect("NouveauMAt.aspx");
                        }
                        else
                        {
                            con.charger("insert into Stocker values('1', '" + txtServiceTag.Text + "')", false);

                        }
                }
               else if (cmbStatut.SelectedIndex == 3)
               {
                   con.charger("insert into Stocker values('4', '" + txtServiceTag.Text + "')", false);
                   Response.Redirect("StockHS.aspx");
               }

                

    }