How do you know when your red ear slider turtle is dead?

Today I cleaned my tank and got a new filer and put my turtle back inside. Then he started to swim around like a crazy turtle and then hide under some rock with his eyes closed and mouth left opened. I don't know if he's in shock or he's dead. He moved once or twice since this way. But he doesn't respond to stimuli. Like when I touch his head he doesn't respond. When I pick him ip nothing. When I touch his legs nothing really... A reflex going in the shell. Has anyone ever had a turtle pass out with his mouth left opened and eyes closed? :(