yep classic symptoms of heatstroke are a lifeless flat ferret who pants like a dog, remove from cage & submege in water (not stone cold )not the head , gently douse the head , place a filled bucket outdoors & you have it there incase you need it, cool said ferret down & make them drink. they recover as quick as they got sick basically
Place wet ferret in a carrier in a cool place with access to water & a towel leave them to recover keeping a close eye on them, just watch as the can also have the shits as well
Hope this helps