The good news here is that everything in your tank and in the world that is currently living has a chance to die, so you are not in this fight alone!
On a more serious note, there are different types of snails (duh, I am a genius!), some which breath air and some that...dont. Apple snails breath air, that I am sure of so if it was submerged for a long time, chances are that it suffocated due to another problem which didnt let it get to an air source.
I do not know much about the pond snail per say, but you would know if the snail needs air because it would climb the side of the tank and use a tube like extention to get air. Some snails also make a bubble of air in their shell so they can float/stay under water longer. This is usually when they are all the way to the top of the water line and stick out of the water a bit. (Important because you need to allow for some space between the water and any glass tops you may have.)
Still, it sounds as if your snail does not move at all and if it is still alive after some weeks, it must not need air very much. Thats good news since you dont have to worry about it suffocating.
Snails are, well lazy. Apple snails will sometimes not move at all during the day and only move at night. They also can appear like they are dead and just float around at the top of the tank but in reality, this is just their way of moving around without expending energy (using water currents). I would never count a snail as dead until its body is hanging out of the shell and obviously not moving at all or the little 'door' falls off from its body. If the little door is shut, chances are that it is holding it shut and is alive.
Some snails also cannot turn themselves over if they are on their sides or upside down. I might try putting your snail upright, with the little door part towards the substrate. If you are really worried, I might actually get myself a peice of cucumber, weigh it down to the bottom of the tank and put the snail, door side down, on top of it.
I know A LOT of people who have the same issue with the lights just popping on and off of a tank. Do you have a single light on the top of your tank or do you have two? If two, like mine, I just put my lights on seperate timers and one goes off before the other and vice versa in the morning. I also have some little LED lights in my tank that are sort of a 'gimmick' type deal, but they do help make sure that there is some light in the tank at all times and so the effect of the big lights coming on and off is less. If you only have one light on the tank, you could put a lamp in the room on a timer that is located away from the tank and turns on in the morning for a little while before the tank lights come on. Then do the same thing for a little while after the tank lights go off at night. (You dont have to have this light on all day, just a little before and a little after the tank lights come on) This will lessen the shock as well. It helped me out a whole lot with my skittish little friends!
