Is my turtle dead ANSWER QUICKLY PLEASE?

this morning he was acting a bit different- he usually hides in his little plant, but today he was casually swimmin around the surface, had his head up out of the water, i smiled cuz i thought he was finally relaxing (hes really scared of everything), but i went to school (7:30) then came home late after my play rehearsal (7:45ish) and he was floating on the top, just chillin (which he does occasionally) but i picked him up and his head and legs just dropped (usually he tries to swim away, lol) and it freaked me out so i dropped him 9from a few inches above the water) and he still didn't move... so my mom came in, we put him on his dry rock, and she was gonna put him in a plastic bag, but she thought he looked like his head was rolling a little less so were not entirely sure... he had a little white spot on the corner of his shell, that appeared one say and never changed, a few weeks ago, i feed him everyday, in his feeding tank, the lights were on all day, as they usually are, and i'm really sad now, please help :(