If you have an electric horn that won’t sound, even though you have tested and proved the electrical circuit as good to go, try blasting spray lubricant into the horn. Oftentimes, a coating of salt or crud builds up on the diaphragm, which prevents it from vibrating. No vibration–no sound.
Really soak the horn, spraying the lube right into the trumpet. Let it soak. Then try the horn. Sometimes a few applications are necessary.
This will not work every time, but works often enough that it is worth a shot before you buy a new horn.