<asp:TextBox Id="mytextbox" runat="server" Visible="false" />
 

If you are trying to read a value from a hidden textbox and you have the Visible attribute set to false, you will most certainly have problems.

To get around this, simply DO NOT set the Visible attribute to false.

Instead, set the style attribute to "Display: None;" like this.

 
<asp:TextBox Id="mytextbox" runat="server" Style="Display: None;" />