You are not submitting the page back to the server, you won't be able to "get" page variables unless you are doing a POST.
change this:
<form id="form1" runat="server">
<div>
<input type="text" id="Hidden1" name="Hidden1" runat="server"/>
<div>
</form>
to this: (EDIT: changed the type="hidden"
too)
<form id="form1" runat="server">
<div>
<input type="hidden" id="Hidden1" name="Hidden1" runat="server"/>
<div>
<input type="submit" value="submit" />
</form>
Then click the submit button