I'm sorry to hear of this incident. It sound slike Zippy is in critical condition. It is impossible to tell if a turtle is breathing. A vet can do that with a stethoscope, but the rest of us just can't tell - the shell makes it impossible.
I'm going to give you a first aid measure to try, and then some information that can help you determine if Zippy is alive.
You can try soaking him. Make a room temperature shallow bath of 1/2 water and 1/2 Pedialyte (made for human infants). Soak the turtle in it for 30 to 60 minutes. Be sure the water is not deep enough to cover the nostrils.
Dead turtles usually have the head and all four legs outside the shell. They often appear limp, and feel limp when you touch them. Eyes may be sunken, but that can also be a sign of dehydration in a living turtle. The eyes in a dead turtle are usually open. The skin of a dead turtle may feel warm to the touch, rather than cold. Give him the soak in Pedialyte and water anyway, and continue to watch for any signs of movement. There's no harm in waiting to be sure. You can also pinch a toe on a back foot. That can cause a reflex motion of the leg, telling you Zippy is alive.
I wish I could give you a more definite answer, but unless a turtle is very obviously dead, it's best to give it the benefit of the doubt and see what happens.
Will you tell me what temperatures you have under the basking light and in the water? Being chilly can lead to extreme lethargy. We can hope that is what happened to Zippy.