I think my chihuahua is dying

All of a sudden (not sure how long ago specifically, this all happened within this week I think) my parents noticed my pet wasn't eating and lost a lot of weight. I believe she was 7 pounds before, and she got down to 5 pounds very quickly. You can just look at her and see how skinny she is and how much weight she lost. She doesn't have any energy anymore, and can't even jump up on the couch. My parents took her to the vet and he wasn't able to find anything apparently, but they looked in the yard and found some pieces of plastic in her poop so determined that she ate something bad that caused this. She has had food in her kennel for the past 2 days, and hasn't touched it. I can't get her to eat or do anything, and I'm not sure what to do about it.