The turtles usually won't bask unless the basking area is significantly warmer (around 10 degrees) than the water. Since you're not sure what the basking temperature is, and since you aren't using a heat emitting light (the flourescent won't heat up the basking area) it's possible that the air temperature has gotten low enough with the onset of winter that they are no longer motivated to bask.
You should provide a heat source for the basking area - a regular household light bulb will do. Be sure to measure the temperature on the basking spot with a termometer to be sure it's warm enough (at least 10 degrees warmer than the water or around 88 to 90 degrees) but not too hot (or it will dehydrate the turtle).
The other symptoms you describe - lethargy and refusing to eat - don't sound good at all. They could be symptoms of a serious ailment. If it were me I'd take him to the vet tomorrow.

