At all big box stores in the plumbing departments or plumbing supply houses, they make a tool designed to go inside the grinding chamber and unjam the motor.
Do not try this with the electric on; this could cause physical harm.
First, check to see if electric is going to the disposal
Second, check to see if there is a humming sound emitting from the disposal.
If this is not occurring, check to see if the breaker or fuse is tripped/blown serving the circuit to disposal.
If the Disposal does not work after going through the above list, I would invest in a new one; you can buy a good insinkerator disposal for around $59 plus tax with minimal problems with the product.
http://www.insinkerator.com
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