Gl gl - ideally they ship you to FRA and get you to usa however possible. I imagine a very limited inventory of services / staff / airplanes for UA in BRU.

depending on your status and your final destination, check any route that exists eg bru-cdr-jfk (af) or bru-LHR-usa (aa / one world if not *A) that you can propose if UA hasn’t already sent you your new itinerary. If needed then check ft UA thread for cancelled flight and related proactive actions to take.