Unplug the unit.
Feel for the reset button on the bottom of the disposal and depress it.
Plug it back in and turn it on.
Listen to the sound produced. If the disposal makes a humming sound, turn it off. It may be jammed.
Remove the obstruction with tongs. Then, turn the knob on the bottom of the disposal to force the jam clear. The knob may require a special tool that came with the disposal or an Allen wrench.
Unplug it and check the fuse and circuit breaker if the disposal makes no sound at all. Reset the breaker if it's tripped or replace the fuse if it's damaged, and test the disposal again. If the electricity is reaching the outlet and the disposal emits no sound, the motor has blown and the entire unit must be replaced.