contents of invisible controls cannot be found in Code behind!

 Dear All,
 
Please help me.
 
I do not know what I have done!  before when a componenet like textbox, or button was invisible in code behind the data could be used!
for example I have a hiddenfield, I want it to be invisible, and in source code nobody can see its content.
OR,
I use a fake button for ajax modal popup!  because I show it by code in code behind, and I do not want to have special target control.
when the button becomes invisible, it is like it is not there! and I get error!  but before it was fine!!!
 
do you have any idea what happend!  how can I fix it?
 
Thanks a loooooooooooooooooooooooooooooooooooooooooot..... :)

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