If your snail is stuck to the glass, it is not always a cause for concern. If a snail is stuck, it is alive because otherwise, it would not have had any grip to attach to. Snails may just be resting on the glass, but sometimes they do that when trying to avoid the water. Water conditions often warn a snail to keep away. That is fine if your snail has been on the glass for a day or two. Snails are not that active, and they might be sleeping or just tired. But if this behavior continues, you must try to get it down by poking the shell gently. Offering more food might prompt it to come down if only to eat more food. If they fall off the glass, then you need to check out the situation.