[RPG] cast Aid again to “heal” creatures already under the effect of the Aid spell that have been reduced to 0 HP

If their maximum hit points is zero then healing magic won't do anything. Healing magic allows a character to "regain" or "restore" hit points, which is always up to your maximum.

When a creature receives healing of any kind, hit points regained are added to its current hit points. A creature's hit points can't exceed its hit point maximum, so any hit points regained in excess of this number are lost.

Basic Rules, Combat chapter.

This disallows cure wounds and healing word and their mass versions, prayer of healing, goodberry (assuming you can feed a berry to an unconscious person) and pretty much all healing potions.

One spell that will help is aid - as long as you keep casting it over and over again.

Your spell bolsters your allies with toughness and resolve. Choose up to three creatures within range. Each target's hit point maximum and current hit points increase by 5 for the duration.

Please note that many effects that reduce maximum hit points also specify really bad things for when the character reaches 0 (for example, the strength drain attack of a shadow). Keeping someone conscious isn't much of an issue when they've turned into an undead monster and are trying to eat your face.

You basically... can’t.

The abstraction that is hp is nebulous and fluid; it’s not an abstraction for any one thing, it’s not even an abstraction for the same thing at a given time. If someone loses hp because a goblin slashes them with a sword, and then someone uses martial spirit to inspire them to greatness, they are healed a certain amount of hp. Here, hp was lost due to injury (probably?) but the damage healed was more inspirational in nature.

The best single thing we can call hp is probably “plot armor,” because that is effectively what it is. That is both its narrative and its mechanical role. Characters have hp because our plots and narratives want them to withstand more punishment than anyone realistically could. Characters survive blows—losing only hp, but otherwise being uninhibited by any injury—because we want to see more of them. And plot armor is always a matter of hand-waving. Good authors can make it more subtle and hide it, but it’s still there because of the demands of the plot.

In short, plot armor is not an effect, it is a cause. The narrative warps to reflect its needs, rather than it reflecting the events of the narratives events. And hp is basically a measure of plot armor—which makes it a little more reflective of narrative events, but only so much.

Finally, stuff like broken bones and sprained ankles—even the lost limbs mentioned by regenerate et al.—just... don’t factor into the system at all. No amount of lost hp causes those things, and those things aren’t defined as costing a certain amount of hp. In fact, those things aren’t defined at all. No effect in the game, anywhere, causes such injuries. As far as the game is concerned, therefore, they cannot happen, and it isn’t the rules’ responsibility to explain how to handle it if a DM decides to houserule things to add them. Instead, that DM is left on their own to handle it, and most DMs simply... don’t bother. Such injuries are either not part of the game at all, or they are matters of pure fiat that the DM is using, effectively, to railroad the players at particular points. You see that kind of thing in video games too—characters that have taken and dealt literally hundreds of attacks suddenly become injured by the plot, and no amount of your usual healing options help—you are forced by the plot to do whatever it is that the plot demands at that point.

A really fleshed out injury system could avoid all of those problems, but D&D 3.5e doesn’t provide one. It is a system that wants to focus on heroic epics—it doesn’t want anyone sidelined by injury, it doesn’t want to focus on the logistical hurdles such an injury causes, it wants to get on with the next adventure.