I'm guessing you have a vinyl helmet(DP) otherwise it shouldn't shift. If that is the case just heat it up with a hair drier or hot water and move it back into place then put it in cool water to set the plastic. Then I would suggest that you put in some place holders to prevent the helmet from shifting in the future.
It's happened to me with one of my fiberglass helmets when the visor was removed. I took a spare piece of abs and temporarily epoxied it inside to keep the mandibles from coming together, worked like a charm.