Flight disruptions are not rare events. They are a normal part of frequent travel, and the difference between a ruined day and a minor detour is what you do in the first ten minutes after the board flips to canceled.
Work two channels at once. Get in line for an agent while you also call the airline or use the app, since whichever connects first wins. Know your alternatives before you reach the front so you can ask for a specific rebooking rather than accepting whatever appears. Speed and specificity are your leverage.
Stay calm and keep records. A polite, informed traveler with screenshots of the options tends to get further than a frustrated one, and documentation helps if you need to claim later.