Well, first check to see if there’s an alternate routing that would get you to LAX in time for your event, if you still want to go. If not, when you get to ORD, explain what’s going on. They may be willing to send you to DTW for no cost. If not, ask them to cancel the last segment, purchase ORD-DTW, and then apply for a refund of the unused ORD-LAX flight, or, if you prefer, for original routing credit for the LAX flight.