Sometimes, the dishwasher can back stuff up into your disposer drain and plug it up. Disconnect the disposer drain and make sure that it is unplugged. If it is in a bind, you can rotate the innards my applying the wrench that comes with the unit to the socket on the bottom side of the disposer. It is an allen head usually.
You can also fill the disposer with hot water and plunge it like you would a stuck drain, but this is really less preferable as there are many ways to plumb these drains, and you may wind up either just relocating the plug to something (dishwasher?) else, or pop the drain line off, making a mess.
IF it is a new disposer, and has never been used, you'll want to make sure theat the casting inside of the outlet tube is cleaned out.
best regards, --W/D--