You have to take it as a shortened form for il n'y a plus de retour en arrière possible. Same as for instance in the very common Pas de problème !. You can drop the 'il n'y a" in this case to add a sense of abruptness to the negation:
J'arrive à la gare: trop tard, plus de trains ! Je sors de la gare, je cherche un taxi: pas de taxi !
The "ne" falls along with the verb, but there will be no ambiguity, the negative form is obvious just from "pas" ou "plus". In spoken French, the "ne" is often dropped anyway: it is very common to hear:
J'ai pas d'argent
J'ai plus d'argent
One might argue that the 2nd example is then ambiguous, as it may mean either I have more money, or I don't have any money left. But plus is pronounced [plys] if if is the comparative for quantities, and [ply] if it is the negation for something that existed before. If you encounter this in written, it will be the context that will determine how "plus* should be understood.
As for possible, it is an adjective that qualifies the noun retour: Possible is placed here after the noun it qualifies. I guess that the puzzling thing here is that it is not placed directly after retour: en arrière is here "closely attached" to retour for its meaning, so it is ok to not separate them by an adjective. You could also say plus de retour possible en arrière - some speakers will possibly say it just sounds "better". If you leave out en arrière, and that you add a full verbal form again, maybe the sentnce will make more sense to you:
Il n'y a plus de retour possible.