I would take him out and rinse him in some dirty tank water first to get all the dead and decomposing snail out of the shell. It will definitely cause a small ammonia spike if you just leave it in there.
Snails shouldn't died that quick from laying on their back one night. You might want to check something else that might be going on. I had mine lay like that for two whole days and I flipped him over and he was fine. Are you sure he is dead??? Smell it, it will let you know for sure he's dead....If he is, gut him and rinse the inside of the shell real good and boil it in hot water. I would let it sit out for a month then put it back in the back. I had to do that with the shells I've collected from FL last summer. It took like 2 months before all that smell goes away. Good Luck!