You
cannot use
FindControl
on HTML controls as long as they are not running on server.
HTML controls are on client side by default and if you specify
runat="server"
to them, only then that control will be accessible on server side.
Thus,
// Not accessible on server side.
<input type="text" id="myData"></input>
//Accessible on server side using FindControl.
<input type="text" id="myDataVisible" runat="server"></input>
If you want to access HTML Control, define
runat="server"
or else it is not possible.