You have 2 choices - rebooking (including re-routing) or refund.

I have the same situation as you - UA ticket (SFO-ORD-MIA). Switched to CO (IAH) to avoid IRROPS. CO then canceled all IAH flights and will not let me to re-route to US (UA can't do anything as CO has the control of the ticket).

Ended up 2 hours with 1P Reservation with limited result (1P Desk ended up using my mile to hold SFO-IAD-MIA F Standard Award to make sure I can make the trip - would reimbruse as much as they can. CO re-routed my to FLL). I was forced to go to the airport without sleeping and tried my luck (CO station chief said absolutely no on re-routing on others, including UA).

Luckily I made the flight to EWR in the last minute (and extra EQM Bonus - CO GA rebooked me on Y and Z).