If the flight from Newark to Toronto was cancelled, you are entitled to a refund regardless of the reason for the cancellation. However, it will be the difference between the Nassau to Newark fare and the Nassau to Toronto fare, so there probably won't be much refund. If the Nassau to Newark fare is actually higher than the Nassau to Toronto fare, you won't get any refund. Go to
www.united.com and in the upper right hand corner click on "customer care". Be specific and concise about what you want, mentioning the refund for the cancelled flight. Be persistent, and good luck. Dealing with U.S. airlines is often an excercise in futility.