On many dishwashers (all as far as I know) the drain impeller is under a little plastic cover right at the very bottom of the internal sump under the filters. Just one security screw holds the cover in place. Sometimes the blades of the drain pump - the impeller blades - get jammed up with bits of debris.
If you can access the blades they should turn freely but with a bit of magnetic resistance from the motor.
You will need a special Torx driver security bit to get the cover off. Tweezers will help with removing the crud.
http://www.askmehelpdesk.com/appliances/screw-bakc-washing-machine-313929.html
Peccavi
January 2011