My 18year old has no interest in anything but sleeping. She won't eat anything for days and throws up bile. Her hind legs are wobbly and falls off balance. I cook her chicken and she eats that sometimes. Please help. She is not on any meds, just old.

18 is old for a border collie. Sleeping all the time is the norm. So are wobbly legs. Throwing up bile means she isn't eating and isn't digesting well. Go to a vet or small pet store that sells special diet food (it's expensive but worth it) and/or feed your dear doggie whatever she wants and can tolerate, because she probably doesn't have a lot of time left. Bland chicken, but also some cream of wheat or rice and veggies with a little oil or cooked egg. Think about if you want her to have to live if she's in pain or if you can let her go sooner. It's always a tough call.

I am going to be a bit surprised if a change in foods will be enough to spark her appetite. At 18 years, there are several serious problems that can be causing this. Any malignancy would not be a total surprise. Organ failures, such as the kidneys, or liver or even heart. Problems start slowly, maybe just a gradual reducing the amount eaten, and progress until eating is just out of the question. Meanwhile the slow progression of the problem has also brought a slow progression of the animals deterioration, until it becomes inescapably obvious.

Perhaps you both may be better served if you have her examined. You will eventually wonder what is/was going on with her, and there is no way to tell without an examination. At this point you are primarily looking for answers, not heroic lifesaving efforts. Get your vet's advice so that you can have a chance to help her, whatever you decide.