Firstly is it still under warranty? If so I wouldn't mess with it. I just took the screw out of the end of the head of mine and peeped inside - I see the spring you refer to, has it come off at the top where it joins the take up bar or at the bottom, where it attaches to the Acufeed thingy? If it's the top, it looks fairly accessible so I'd try myself. As long as it wasn't under warranty. One thing though - there are wires in there - switch off and unplug first, don't electrocute yourself!
PS If it's happened twice, perhaps mention it next service as there may be another reason this is happening - has the spring got stretched for eg?