Most disposals have a reset button on the bottom. If there is no sound when you turn it on, check to see it the button is popped out.
If the disposal makes noise but wont start turning, it is probably jammed. Often you can fix this with a wrench, turning the socket in bottom of the unit. Many disposals come with the wrench needed to turn it, or by manually turning the grinder with a short broomstick you can often get it working again.
Be sure the power is off before attempting to fix it!
If the grinder makes no noise, it could be no power. Most kitchens have GFCIs and depending on how the disposal is wired, resetting this could fix the problem.
Sometimes there is a dedicated breaker in in the panel, check to see if it has been tripped. This is where you should turn off the unit before attempting to un-jam it with a wrench or a broomstick.