What can a PhD student do when a professor is no longer interested in advising them?
I would urge you to not lose hope and keep an open mind about your research interests. If your field is more applied than theoretical, it is very much possible to find advisers from other departments who might be doing the same flavour of work. Contact everyone you can! Talk to other students and see if any of their advisers has funding. 

As far as teaching assistantships go, often times departments take applications from students outside their own (this is true particularly for departments that get plenty of research money, example, some engineering ones). Don't hesitate to reach out to the staff in those departments!