Yes: Occasionally this can happen. It may be because of excess moisture in the wormery. Excess moisture by itself shouldn't hurt earth worms, but the problem may be lack of oxygen related to high moisture levels. It may be because of decaying food. Mouldy bio-waste in itself also shouldn't kill earthworms. But should you find mouldy food leftovers in the wormery, remove them. It may possibly be that the worms are starving. Earthworms may starve and die after only several weeks without a food supply. If you managed to compost your worms, don't panic. Try again with a new hatch of earthworms!
Alternatively if you feel the need to start again, our Wormery Rescue kit has everything you'll need to give it another go. https://www.wigglywigglers.co.uk/wormery-rescue-kit.html