Iv'e seen many posting on widget disappearance to put it lightly. I'fm having the same issue and I am horrible at PHP.
Basically I have a widget, the archive one of all things that won't show up but the subscribe widget will. I'm using I think the twentyeleven theme but I cant' remember as it's been about a year since I checked but that's what I'm sure I'm using. the website can be found here. http://tffp.marrie.org. and here is the code to my function.php file
I know I'm missing something simple here but I cannot put a finger on it. thanks.