in an MMO, the tank's (a job/character type that has high defense) role is to soak up the damage and attention from enemies so the DPS (damage dealers) characters can fight w/o taking much damage to their own selves while the healer keeps everyone topped off
if the tank is gone, DPS and healer are gonna wipe, which I've had happen to me in FFXIV before
I've also had the healer disconnect in a fight, and we wiped there too, but everyone was really kind about it