I have a 2 year old St. Bernard and she won't eat. I recently had her boarded and she ate then, but after a couple days back home she won't eat. What is causing her not to eat?

Change of diet? Upset/worried about being left again? Sick?
Is she drinking plenty of water? Does she have diarrhea? If you answer yes to more than two of these questions maybe it is time to go to a vet. She may have picked up some bug at the kennels.