How to proceed with nonpayment of a contract for work performed

I had a consulting job this summer and worked 43 hours. My contract stated I would perform marketing analysis tasks, which were not specified, gave no end date, did not outline specific deliverables, etc. and said I would be compensated an hourly rate. I conducted marketing surveys according to the format they asked me to use and turned them in when I could no longer collect additional data. They claim that I was not only "technically" late, but that the information I provided wasn't useful and they wouldn't be paying me for my time. I received a small stipend in exchange for my "efforts" that was less than 20% of the total for the hours I worked (and documented). Never was there feedback about when they were due or that there was any issue with the sample survey results I sent in. I think they are in breach of contract and I want to know how to proceed. I have not cashed the stipend as I do not want them to think I have accepted it. Please help. I'm a medical student and don't have many resources at the present time. I can give good karma! Thank you.