How high is the tank's water level? If you keep Briggs, they lay eggs on top of water. Your snail may have been trying to lay eggs if your water level was too high, so it ventured out of the tank. Probably got lost along the way and died. Aside from that, mystery snails don't normally climb out of the water for fun, like nerites do.
To avoid this, get a cover for your tank or lower your water level about by about an inch. Just enough room for them to get out of the water for a short while, lay some eggs, then hop back in without actually exiting the tank.
Sorry for the loss
