Oof, with travel within the next ~72 hours and with the trip already having failed to ticket once through United, you may just want to accept the extra booking cost of going elsewhere and use an online travel agency like Expedia, Orbitz, or Opodo, who may be better equipped for this.

If United tries to ticket the trip and can't make it work, they're probably not going to be much help in getting you to your final destination.

You should definitely be sure to check, as WinecountryUA says above, that your trip has actually been cancelled. Plug in your United record locator (six-character code from your reservation confirmation e-mail) and your last name at united.com and see what it says about your reservation status. If it says "there are no segments", the phone rep was probably right. If not, the phone rep might have been confused.