It is likely that there is a penalty associated with the ticket. Typically it would be $200 for a domestic ticket and $300 for an international ticket.
Many US tickets also require that the ticket be cancelled prior to the scheduled flight. If you don't, there may be no value.
Rather than torturing yourself, call US directly. If this can wait until the blizzard in the midwest/northeast is resolved, you will find it much easier to get through to US.