I've had that problem in the summer when they get too hot in transit - or when they get too hot after they arrive. They also die off like that if they are too crowded.
If you're an experienced mealworm farmer you've probably already crossed off temperature, food or humidity(drowning). Those are the common culprits for sure. Any chance you recently sprayed the room with an air freshener, or maybe cleaned some cages with bleach or another harsh chemical in that room? Did the neighbors recently bomb their house with pesticide? Etc.
The only things I can think of is possible contamination by chemicals, or else "something" got into the cage and killed them but couldn't eat everything it killed, i.e. a mouse or rat. I've never had them all just drop dead from saying "boo", there's usually a reason even if it's as silly as accidently hitting them with hairspray while doing my hair. They're pretty sensitive to chemicals.
wow, theres one thing that sticks out to me. over crouding, and i bet thats what it was. i never had this problem befor, but ill lay money on it, that was it. thanks, and ill have to be mroe carefull, to see if that was it