I'm in pain; what's the cause? Stomach ache? Cut my foot off? Hang nail? It's impossible to guess without some preliminary diagnostic information.
Are there any other clues or symptoms? The air bag computer detected an electrical problem with the system, disabled it, and turned on the light to let you know about it. The mechanic will connect a hand-held computer called a scanner to retrieve the stored diagnostic fault code(s) that will lead him to the circuit with the problem.
The most common problem is the " clockspring" under the steering wheel. It's a wound-up ribbon cable that eventually breaks on the ends. The horn and cruise control also have circuits in the ribbon cable, so they will also be affected eventually. If you try to replace the clockspring yourself, be aware the tires and steering wheel must be straight ahead before you install the new one. If the steering wheel is off-center by one turn when the unit is installed, it will either become wound tight and tugged to where it breaks, or it will unwind so far it folds over on itself and will break soon from the hard flexing and bending.
Look for other things that don't work. There are always two fuses for the air bag computer. One provides power to turn on the warning light if the other one blows. It is common to run safety items off an air bag fuse to get your attention. For example, if the horn fuse blows, you would never know it until you needed it.
The air bag computer constantly monitors the integrity of the wiring to the impact sensors and the air bag itself. Problems with these circuits will set the appropriate fault code into the computer's memory.