Rose - You should put it in a smaller container of water with just enough water to cover it. This may help it get its footing and be able to stay right side up. Keep the water clean and cool.
If this does not seem to be helping it you could try very gently massaging its belly. Do not over do it, touching it will rub off its protective slime coat.
Could you tell us how long you have had it, what you have been feeding it and how long has its tank been set up?
Do you have a test kit so you can test the tank water?