Help wih my betta fish?

i have a betta fish and he is not moving alot here lately is he about to die or something he is 2 years old and he is healthy well thats whats the pet store said they said that fish are like human and can catch colds if the water is 2 cold so i put i light over the water to keep it room tempurature but 3 days have ped and he is stille at the bottom of the bowl barely moving but still alive has anyone ever had this problem if so please tell me what is gonna happen so im prepared