Gary Miller was closest. You can also try this:
After the outlet hose has drained, open the dishwasher door and blow hard into the outlet hose (try not to get any gunk in your mouth! I usually put my hand in between the hose and my mouth.
You will find a bit of resistance then it should clear. Remove the filter (the one you remove and clean after every wash) and see if there is any gunk in the sump. Remove what you can by drawing the water out as best you can and using a rag to remove the gunk. Close the door and see if it tries to fill. Let it fill for a while until there is some water in the sump. Do the blow through the outlet hose trick again to see if it has cleared.
If you ket the dishwasher drain via the outlet hose, any gunk in the hose will end up clogging it again.