Annie found this, hope its of some use
You can try two things - placing her in a warm, dim place where she can sit and relax. The key is to relax her muscles so she can pass the egg.
Or you can try standing her in a bowl of warm water, which she'll drop her vent/cloaca into, and this can help relax the muscles. I've heard this method can be pretty succesful, but please make sure you dry her off thoroughly before she goes back outside.
If neither of these work, you can go to a vet who may try to syringe out the contents of the egg, then remove the shell bit by bit. However, as soon as the egg breaks or is broken, her chances reduce dramaticaly as the chances of infection are high. So it is best to try to get her to pass the egg herself.
At the same time, this is a critical situation, to speed is of the essence.