It's very concerning when they don't eat isn't it?? My Chi, Bonnie, went without any food for two days late December and wasn't her usual happy self at all.....she lost lots of weight - it was terrifying.
I took her to the vet and there was nothing physical wrong with her. This might sound strange, but I think it was MY MOOD at the time that was upsetting her...I lost my mum to ovarian cancer on New Year's eve 2004 and wasn't at my happiest leading up to the anniversary recently. Bonnie must have picked up on my mood - they are such clever/perceptive little dogs!! Since then I've seen her watching, and I mean really watching my face at times - she is my "Happy Police".
Of course, there are normally physical reasons for them not eating, but just thought I'd share my unusual experience..... BTW, she's eating fine now and is looking decidedly plump!