Are you sure it's on the top? Does it look like this (under the snail in this picture)? If so, this hard brown disc is the snail's foot, on the bottom. If you're seeing it on the top, your snail is upside down.
If it stays that way, you can reach in and set it right side up. They can have a hard time righting themselves. They cover a lot of ground, and sometimes use falling as a shortcut to get somewhere else, landing on their sides or top in the process.
If it keeps returning to upside down, it could be a sign that it's heading for the grave (sorry). Is it big? it could be old, and nearing the end of its natural lifespan. Anyway, if it keeps returning to upside down, try putting down some food and righting the snail on top of it. This would give it an interest in staying right side up, maintaining nutrition, and continuing to live.