Peter's wiki link contains the answer. I had a similar problem in that my handle became loose and didn't connect to the drive that operates the mech. I couldn't open the box using the central hole. Instead, I had to get a strong piece of twine (poly cord; the sort of stuff that parachute lines are made from) and then using a coat hanger I threaded the cord through the operating mechanism on the side (the final para in the Wiki post). It took a bit of doing, but I managed to threat the cord through and open the lid eventually. Study a few photos of the mechanism to understand how it works before you labour away. And if you can open it using the central hole, that seems to me to be ideal. My mech was too stiff.
I have left the piece of cord threaded to the side mech - just in case this happens again.
Enjoy!
JH