i think youll find the frost will have been there before the fault occured, you probably just didnt notice, as the appliance works on the principal of evaporation/condensation, you will get condensation on the back wall (which is where it freezes) defrosting itself, and draining away through the drain channel at the bottom (which is why you will sometimes see water, instead of frost on the back wall)
dont worry, its fine.. have a great xmas