Some people don`t even open their freezers for several weeks until they smell like something went wrong. This is why it is recommended to buy a cheap wireless thermometer that will beep if the temperature in the freezer is not sufficient. But if this already happened – immediately disposal the food from the freezer, pure concentrated bleach into the freezer (may be on the walls also), so you could use it the future. Turn the freezer off, to keep the  compressor of the freezer from damages,   and call a professional appliance repair service (the one you can trust)> it can be just a  manor fix..