Dog Has completely stopped eating.....?

I have an 8 year old boxer. She has always been an active playful dog, recently she has stopped eating and she's not active or playful. Its been going on for several weeks now. At first she stopped eating the dog food, we tried two different brands of dry food and also can food and dog cookies. She would sniff the food but that's all. After a week we tried giving her other food like rice, tuna, sliced ham, sausages, chicken broth, etc. At first she would eat that stuff but then she stopped. We've tried this paste that opens your appetite but it hasn't helped. We took her to the vet, where they did two x-rays to see if there was any blockage and there was none. They also did blood work checking for worms, her thyroid, liver and kidneys. Everything came back normal. She is getting very thin and doesn't eat anything at all and does not drink water either. What can we do??? Its horrible to watch helplessly while she starves herself.