That's what you pay mechanics to diagnose. There's crash sensors, initiator circuitry, a computer, and all the wiring between them. Luckily the computer will set a diagnostic fault code in memory to indicate the problem it detected. Those codes do not tell you which part to replace. They only indicate the circuit that needs further diagnosis. Your mechanic will read the codes, do some preliminary diagnosis, then give you an estimate for repair. The most common failure is a broken clock spring under the steering wheel. As it continues to break, other circuits that go through it will also stop working. That's usually the horn and cruise control.