Sounds like it could have been damping off, which can be caused by a number of bad fungi (including fusarium, botrytis) that like warm, humid conditions. Cannabis seeds really don't need a dome to germinate\grow and I wouldn't recommend using one, unless perhaps you live in a desert. Even then you would at least want to keep the vents open. Cuttings absolutely require a dome initially, but again ventilation should be introduced relatively quickly (after a few days).
Seeds are best started in media in a container at least 4" tall to accommodate the taproot (seed goes in about half inch deep). It just needs to be moist, dark and warm enough for the seed to want to sprout. It shouldn't be saturated\wet at all times as this and high humidity can certainly lead to premature death in this manner. Sometimes the seed doesn't even germinate, but keeling over at the soil line is the classical sign of damping off.
