Snails are both male and female at the same time. Mystery snails breed and then lay a cluster of creamy white coloured eggs just above the water on the coverglass or plant leaves.
To see if your snail is alive, touch the operculum (the trap door they use to seal themself inside their shell). If you gently pull the operculum and the snail does not instantly pull it back and try to close it, the snail is dead or dying.
If the snail is unwell or dead, then something is wrong with the water. It could be a chemical like copper or fish medication, or ammonia or nitrite from a filter issue.
How long has the tank been set up for?
Does it have a filter?
How often do you do water changes and how much do you change?
Do you use a gravel cleaner to clean the substrate each time you do a water change?
Do you dechlorinate the water before adding it to the tank?
Have you put anything in the tank in the last week, medication, ornament, anything?
Check the water chemistry for ammonia, nitrite, nitrate & pH and post your results here. If you don't have test kits take a glass full of tank water to the petshop and get them to test it. Write the results down when you get them and post the results here.
I would also do a 75% water change and complete gravel clean now. Make sure any new water is free of chlorine/ chloramine before it is added to the tank.