There is a real possibility that the cat was malnourished when she was pregnant (and even before). If a pregnant cat (or any mammal) cannot get enough nourishment for her fetuses to develop, then she will sacrifice her own body tissues to support their development until she has no more to give, and the fetuses either abort or are born prematurely. Also, since the mother is severely malnourished, she will not be able to produce milk and her kittens will starve.
If you cannot get her to a vet for diagnosis, then the only alternative is to provide her with as much food as you can. This will mean animal protein and fats. Vegetables and other non-meat products will not do. Neither will she be able to digest cow's milk (or other milk products), since cats are lactose-intolerant. In fact, cow's milk may make matters worse, since it will only cause diarrhoea.
If you cannot provide complete (rather than complementary) cat food (e.g. tinned cat food), and preferably as much food as she is willing to eat, then a diet of mice (or other small mammals) is a possibility (providing they have not been poisoned) if you can catch them (she will not be able to). Butcher's scraps are another possibility. I'm sorry, but I don't know what area of the world you live in, so I am trying to cover all the bases.
If an adequate diet helps her to recover, then you need to think about getting her sterilised, otherwise she will just go through all this again if she gets pregnant and you stop feeding her.
This is non-veterinary advice, but it's the best that I can give you. I am a cat owner.
If anyone else has advice to offer, please feel free to edit this answer. Alternatively, leave comments and I will incorporate them into my answer.
