I replaced the timing belt, tensioner, water pump, and all associated parts that come with this job March 2020 at 270,000 miles. Now it is June 2021. I was driving the car on interstate and the car killed. I towed it home. Upon inspection I've noticed the timing belt is broken. The car currently has 294,000 miles.
First of all I don't understand why the timing belt broke after 24,000 miles. Can I replace the timing belt and move on or is the engine toast?

 

where did you buy the timing belt? I don't know of the belts breaking unless they were non-oem or some other critical part was installed improperly or broken; in the second case there would be some noticeable noise that had to have been ignored before the belt broke.

You could always replace with new belt and perform compression test to confirm status of affected internals.