My 4 - 5 year old cat has no teeth?

My cat has no teeth at all! Shes 4 - 5 years old, She was left by our doorstep one day. my mum said she has no breed? and we only feed her human food. We don't take her to the vet because it costs too much and now my cat is Really skinny, When i pick her up i can feel her bones and she also can't meow or anything. I'm really worried. What should i do?