Usually, that means you had an error in the syntax of the function that you ignored.
Once a function has a syntax error, it is non-existent from other code's standpoint. Hence undefined.
(And I'd bet you are using MSIE, no?)
LogicAli beat me this time.
Anyway, learn to use the debugger in your browser!
If you are using Chrome, look here:
The MSIE debugger is very similar, though just a tad less user-friendly in my opinion.