How do I close fancybox from C# (code behind)?

I created a fancy box. It has a button like this - 
<input type="button" value="Cancel" onclick="parent.$.fancybox.close();" />
When the user clicks on 'Cancel' it calls - "parent.$.fancybox.close();".
But, I want to call "parent.$.fancybox.close();" from my C# code (code behind).

Any clue