The crop is probably dry inside and the food dried into chunks posibley cloging the crop passage way, I would take your bird to a vet ASAP! because it can die from this if the food does not go threw.

Try adding a bit water into the crop too soften it up!

Join my pigeon site, http://www.pigeons.biz/forums/forumdisplay.php?f=10

They'll tell you what to do.