Ms. Bell:
Check the terms and conditions of your ticket (and United's contract of carriage). The industry norm for a cancellation penalty of a non-refundable ticket is now between $100 and $150. With the balance being a credit for future travel to be used within 1 year of either the travel date or the booking date. (depending on the carrier).
In regards to United "not accepting" your complaint. I would like to give the benefit of the doubt to United and say that it might have actually been a technical issue that has since been resolved. But if you would like to ensure that they recieve your complaint, file one with the DOT at
http://airconsumer.ost.dot.gov/problems.htm A copy of your complaint will be forwarded to United Airlines.