It is hard to find dead worms in a worm box, but they do die in the box. Dead worm bodies decompose very quickly because their bodies are between 75%-90% water.
If you find many dead worms you should find out the cause. High heat (above 84 degrees) is fatal to them. Too much salt or acidic food waste can kill them.
It is best to change the bedding with fresh materials to solve the problem. Sometimes, partially replacing bedding may solve the problem.