Value in a dropdownlist is changed on page postback

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C#

ASP.NET

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Hi all,
I have a Gridview with a checkbox.The Grid is loaded based on values of two dropdownlist. The values in a dropdownlist is loaded based on web services.

Now my problem is when the checkbox is selected the page is refreshed and the values in dropdownlist will be set to default.That is, in any case when the page is refreshed the dropdownlists value is set to default. Only the dropdownlists value is changed,other fields value remains the same.I tried using updatepanel but it didn't work.

Any help will be really appreciated.

What I have tried:

<td>
                                   <asp:DropDownList ID="ddl_Prov" CssClass="txtboxAll"   Width="260px" runat="server" ></asp:DropDownList>
                                                  <asp:CascadingDropDown ID="CascadingDropDown1" runat="server" Category="facil" TargetControlID="ddl_Prov" PromptText="[Select Group]" LoadingText="Loading  Group..." ServicePath="~/Services/eservice.asmx" ServiceMethod="GetDropDownProgroup">
                                                  </asp:CascadingDropDown>

                                </td>

<td>
                                     <asp:DropDownList ID="ddl_Pro" CssClass="txtboxAll"   Width="260px" runat="server" ></asp:DropDownList>
                                                    <asp:CascadingDropDown ID="cdd_fa" runat="server" Category="facility"  ParentControlID="ddl_Pro" TargetControlID="ddl_Pr" PromptText="[Select Pr]" LoadingText="Loading Provider..." ServicePath="~/Services/Freservice.asmx" ServiceMethod="GetDropDownFacilitybyGr">
                                                    </asp:CascadingDropDown> 
                                  
                                  </td>

<asp:TemplateField ItemStyle-HorizontalAlign="Center" ItemStyle-Width="3%">
                                        <HeaderTemplate>
                                        <asp:CheckBox ID="chk_All" runat="server" AutoPostBack="True" oncheckedchanged="chk_All_CheckedChanged"  />
                                        </HeaderTemplate>
                                        <ItemTemplate>
                                        <asp:CheckBox ID="chk_select" OnCheckedChanged="chk_select_CheckedChanged" AutoPostBack="True" runat="server"  />
                                        </ItemTemplate>
                                        <ItemStyle HorizontalAlign="Center" Width="3%" />
                                        </asp:TemplateField>