This means you had a jQuery conflict. You could have seen that by pressing Ctrl+Shift+J when using Chrome.
This happens when another script (for example Prototype) is also using $ shortcut or when you are loading two instances of jQuery, or... (There are other cases too :P )
I am noting this, because you may have the same conflict on your other pages too.
b) If you cannot avoid using other libraries:
replace it with:
By this fix, you don't need to replace all $'s with jQuery's.