It is in a pseudo hibernation, mine are doing the same thing.
All turtles go through this and you can stop it but it isnt healty for the turtle.
Best thing to do is increase the daylight hours, warm up its tank and the turtle will start eating again. Personally I would let it go through its hibernation it will only last a short time and it is benificial to the turtles health.
remember live crickets, meats etc will foul the tank quickly, be sure to remove any uneaten product for the tank.
You can put a filer on any turtle tank to cut down on a lot of the cleaning.
I run a 350 ga pond filter,( flugal puts out a great one) on the indoor pond and a whisper 20 (amended to reach the bottom of the tank) on the 10 gallon with the young turtles in it.
clean water is essential and all the water changes and cleaning can be down right frustrating and time consuming.
Your turtle will be happier with a filter and you will be happier with the extra time to do other things.
Yellow bellys are nice turtles I have those in with the red ears.
Most fo the turtles here are dump offs, turtles to big for owners to handle, I hate the " aww its so cute when tiny but the minute they get over 6 inches they want them gone.