If you turtle has gone off it's appetite, there could be a number of reasons for this. One could be that the water is too cold. Some turtles won't eat when the temperature goes below 20 degrees celcius. Almost all turtles will stop eating when it drops below 18 degrees celcius. Checking the temperature would be a good start.

If the temperature is OK, another reason could be stress. If you have recently moved the turtle to a new 'home', it may not eat for a few days until it settles in. If it is this, it's probabaly best to leave the turtle and let it adjust itself.

Another thing could be that it is actually ill, for example, with fungus or a cold. Diagnosing a turtle's illness is not something that is so easy. Probably the best thing to do would be to ring around and find a vet that actually can treat turtles (as not all can). At least this way you will know for sure what it is and they will be able to treat it.

Good luck and hope your turtle gets better soon!