Either they have run out of food or the conditions in the bin have become unsuitable for them. Worms hate waterlogged, acidic compost. Piling in a thick layer of kitchen waste so that it begins to putrefy and exclude the air will cause this sort of problem. Adding fresh green materials that heat up as they decompose will also kill worms or drive them away. Plastic worm bins do not always allow enough drainage from the compost; make sure that liquids are not collecting in the bottom of the bin to flood the compost.