Most disposals have a "reset" button on the bottom or side of the unit which acts as a built-in circuit breaker. Depress this button to reset the unit.
If that does not help, try resetting the designated circuit breaker on your electrical panel.
If the disposal makes any sound at all (i.e., buzzing) when turned on, it probably means the unit has power, but is jammed.
Use an Allen wrench to clear the jam. This is a five-sided, L-shaped tool that will fit into the bottom of the disposal ($2-$3 at a hardware store). With the unit turned off or unplugged, insert the Allen wrench into the bottom of the disposal. Move it clockwise and counterclockwise to loosen the jammed unit. Sometimes you can reach into the disposal and remove the stuck item if it is large enough. Always unplug the garbage disposal before reaching into the unit. Once you believe the unit to be clear of the jam, plug the unit back in and turn the disposal on to see if it rotates!