Some breeds of hamsters do hibernate as a response to cold or lack of food/water. Unlike most species they don’t however "fatten up" before hibernation so it can be quite dangerous. Assuming this is what your hamster is doing then I would not recommend leaving him to hibernate as he could become dehydrated. I would move him into a warmer room, then gently rub him with a warm towel once he has warmed up.

If he is hibernating then this should rouse him a little bit. You then need to feed him sugary water (one teaspoon per cup of tepid water) with a dropper or however he will take it. This should perk him up enough to start eating normally again. You may need to move him into a warmer area (or get him a heating blanket) if he is hibernating or it may well happen again. If this treatment does not wake him up or he appears in pain then you need to take him to the vet.