Hi, it could be many things. If you just acquired the turtle it may take a week for it to get used to its new surrounding before it feels comfortable. If you have had it for awhile , check for shut eyes and lethargic behavior, which would indicate illness.Hope that helps!
It could be ill
It could be that he's not used to being fed yet or it could be that hes hibernating. It could also be that he's sick. Check his eyes and skin regularly.