I am curious: you state you signed a contract for a non-paying job. In order for a contract to be binding there needs to be an offer, acceptance and consideration-both parties need to be giving up something. What were you being provided in return for your work? Exposure? Was there any language pertaining to their right to fire you in the paperwork that you signed?
Valid contracts are binding. There may be continuing obligations from you to them, and perhaps from them to you as well. I would need to see the actual agreement you signed to see what obligations and liabilities may have existed prior to termination and subsequent to termination. I have drafted or consulted on numerous entertainment and film related agreements over the years.
There are some missing pieces in your story, not the least of which is the nature of your contract. The fact that your employer terminated you and is now threatening legal action (for what?) sounds contradictory, but more facts would explain how this came about and what your rights and obligations are.
This answer does not establish an attorney-client relationship. It is for informational purposes only. It is not intended as specific legal advice regarding your case, and the answer could be different if all of the facts were known.