If we are speculating that something has broken, my guess would be a broken balance staff actually (in particular if this one is old enough to have a non-shock protected balance). Given the small mass of the escape wheel in relation to pivot diameter, a broken escape wheel pivot would be much less likely I think.
Of course a broken mainspring is the easiest thing to test - just wind the watch and if it comes to stop as normal, the spring isn't broken. Unless it's an old blued steel mainspring, the fall should not have caused damaged to the mainspring.
But there are other possibilities that are less drastic to repair. One would be hands getting bent from the fall and jamming, but that should be easily visible. The other less visible one would be the dial feet are bent from the impact, and the dial has shifted relative to the movement and is binding on the hour wheel, stopping the watch. It could even be a tangled balance spring - caught on the stud or regulator for example.
Cheers, Al
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