If the 2010 Corolla's horn is not working, what is the cause?

There could be multiple causes that would result in the horn on Your 2010 Corolla to stop working. Let's start with the most basic: check the Horn Relay fuse; this should be labeled in the fuse box in the engine compartment. If that checks out, the next thing to look at is if there's a bad horn switch in Your steering wheel, where it's not becoming engaged when You press it. In this same train of thought, You could also have a broken "clock spring" under the steering wheel. Finally, a broken wire or corroded ground can also result in a non-functioning horn. Verify that all the wires to it are in condition and functioning.