Salonerosa, before I start nattering on, here is a thread that addresses the same problem: LEA wont eat her croquets :(
There are many, many causes of a dog suddenly refusing food. We would just be guessing given the information you have given, so perhaps you could read through this thread, see if anything discussed there applies, then come back with specifics.
Some of the things that are helpful to know are: what food/brand/ingredients do you feed, how much do you feed, how do you feed, when do you feed, what is your dog's output, drinking behavior especially changes, etc. Also relevant to the discussion are any behavioral issues, any sudden changes in behavior, any recent bad experiences and/or changes. And of course, first and foremost, any medical concerns, general health of your dog, have you seen a vet recently, have you recently wormed the dog or given anti-parasite meds, etc.
So take a browse through that thread...