In dnd when you go to zero hit points you go unconscious. You then have some chances where you stabilize yourself or die. If you’re healed even 1 hp during that time you become conscious again. If you die your character can only be brought back to life with high level expensive magic. Revivify is one of those spells and the DM cancelled it with counterspell. This consumes the expensive spell and they possibly might not have another ready to go leaving the probably high level barbarian dead. The player would need to roll a new character for the campaign. Since they’re fighting a dragon that’s powerful monster they’re probably pretty high level which means this campaign may have been going for months or even years