lethatgic 4 month old lab

Question
Our 4 month old lab stopped eating or drinking the past 12 hours or so. He normally eats very well and is very playful. But in the past 12 hours or so he refuses to eat or drink and is very lethargic.  I haven't noticed any vomiting or diarrhea( he is an outside dog). What could be wrong. Does this warrant a trip to the vet.

Answer
Sorry it took me so long to answer but I had an emergency crop up.
Anytime a young dog with questionable vaccination history (to me as you did not include it) gets like this I worry about parvo of course and even distemper.

You are not sure if there is any vomiting so I would bring him in the house and observe him to see if he is eating, vomiting or has diarrhea.
If he has any of the above you need to call the vets (not for eating of course, but for not drinking water).

Time is of the essence if he indeed does have parvo or gastro-enteritis.
If he is vaccinated he can still get parvo but has a better chance of making it out the other end ok.
I hope you got him into the vets already if he is that sick.
Please let me know.