Hello and welcome to the forum family!
I'm so sorry to hear you're having problems with your snails. If you could post the stats for your tank it would help us to solve your problem. IF you know it, it would be helpful to know:
2. Ammonia level*
3. Nitrite level*
4. Nitrate level*
7. Tank size
8. Other tank inhabitants (fish, snails, and others)
9. Recent tank history (e.g. is this a newly established tank? When was the last water change? Temperature fluctuations? New equipment?).
10. Any fish medications or other additives used in the water in the recent past.
They don't have to be floating to be dead. If you gently pull on their operculum (trap door) do they pull it back tightly? If they don't smell like the worse thing ever, then they still may have a chance. We look forward to helping you!