If your garbage disposal doesn't start when you turn on the switch, the first problem to suspect is a tripped circuit breaker or ground fault interrupting receptacle. If the disposal won't come on after you've reset the GFI or breaker, there's probably a clog, and the internal breaker tripped. That can happen if you're in the habit of putting bones, coffee grounds or fibrous vegetables in the drain, and it's usually not difficult to fix. If it's not either of these problems, there's probably something wrong with the motor or the connections.
Check the GFI, then the Breaker
If your garbage disposal is a plug-in model, either the GFI outlet into which it's plugged or the circuit breaker in the main panel could have tripped.
Reset the GFI
All GFI receptacles have a pair of buttons; the top one is the reset button. It's larger than the other button and, if the buttons are colored, it's red. When you push it in, you should hear a click if the receptacle was tripped. If nothing happens, it's possible that the breaker for the circuit tripped instead.
Reset the Breaker
Locate the breaker that controls the garbage disposal circuit in the main panel; it should be labeled, but if it isn't, just look for a tripped breaker. You can tell that a breaker has tripped because the toggle is offset from the others, and on some panels, you'll also see an orange warning flag. Reset the breaker by turning it all the way off, and then turning it all the way on. It won't reset if you simply turn it on without first turning it off.
The Breaker Didn't Trip and I Can't Reset the GFI
There may be more than one GFI on the circuit controlling the garbage disposal, and if there is one closer to the panel that tripped, it's cutting power to the garbage disposal. That receptacle is probably also in the kitchen, but it could be outside, in the bathroom or in the basement, depending on how your house is wired. After you reset that GFI, you should have power in the garbage disposal receptacle.
Hardwired Garbage Disposal
If your garbage disposal is hardwired, you don't have to worry about resetting a receptacle. Simply check the panel for a tripped breaker, and if you don't find one, the garbage disposal is probably clogged.
Clear the Clog
If you can't find any problem with the receptacle or the breaker, the garbage disposal probably clogged and tripped the appliance's internal breaker. Turn off the breaker or unplug the garbage disposal before you clear the clog. Once the rotor is turning freely, restore power and push the red reset button on the side or bottom of the canister to the appliance.
It's None of These Things
If you can't reset your garbage disposal, it's possible that the wiring is loose and something got disconnected, or something is wrong with the motor. Either way, it's time to call a service technician.