maybe you could tell us more about your settings....
water tempareture? basking temperature? size of the tank? how often do you feed the turtle?
any turtle over 2 inches should only be fed protein (aka pellets, worms, other bugs) every other day and ONLY enough that should fit into his head.
also your water temps should be in the 70's and the basking temp should be about 85 degrees. if the temps changed lately, this could be a reason that your turtle is not eating. it could have gone into a short period of hibenation because of a fluctuation in the temperatures.
but turtles can survive a LONG time without food.... so do not worry if everything about your set up is perfect! they can survive months without food. Most people over-react when their turtles stop eating, but it's quite fine and sometimes even natural.
hope this helps 
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~ Evie 
Pets:
RIP 0.1.0 German Shepherd {Dusty}
0.1.0 German Shepherd/Collie mix {Shadoe}
0.0.2 RES turtles {Booger & Snot}
1.0.0 African Clawed Frog {Bingo}
0.0.2 Snail {Gary & Larry}