Object reference not set to an instance of an object

I call a function on my development server and it works, but when I deploy it on live it does not find it and gives the above error! I can't think why, when I empty the function to do nothing at all just exist, it still fails.

So its not the contents that are causing this, just the fact that the function cannot be found. Almost as if the new dll is not being used.

Any help greatly accepted.

RichardPlease check if you had placed the dll's in the correct folders or not
Thanks,

Only one dll, and it goes in the bin directory, any other ideas?