Check the water quality for ammonia, nitrite, nitrate and pH.
How long have you had the fish for?
How long has the tank been set up for?
If you have only had the fish or a few hours, give it a day to recover from the trip.
If you have had the fish for a few days and it still sits on the bottom, do a 75% water change each day for a week and see if that helps.
Make sure any new water is free of chlorine/ chloramine before it's added to the tank.
If the water is good and there's no improvement after a couple of water changes, let us know and we can investigate further.