they are def stressed.is there any white slime around the edges of the shell opening?
if so remove all & put them in a bowl of treated tap water until they return to normal.( if this happens on every large WC,then use another water source for recovery)
having killed my share of mystery snails they look like less than 24hrs till death.
what changed with your water supply?
copper traces will do that in under an hour.(dead in 6)
if cause is unknown place them in a bowl of bottled/distilled water until you figure out the reason.(where you may/not see the white slime)
a large temp swing during water change can do that but it would need to be greater than 5-8C.
floating is their fight/flight response for survival,float to surface & allow wind/current to move them rapidly away from present undesirable location.
the ones on the bottom remove those 1st ,smell the opening of the shell you'll know instantly if its dead. smell the rest as you remove put those in a different container.(unlikely any survivors in the smelly group)