Tout is the singular form of tous.

Tout s’est bien passé

That’s what it would be. For the plural form, it would be something like that :

Tous se sont bien passés.

Talking about exams, for instance.

The feminine form of tout is toute. For tous, toutes.

Edit : As some people cannot understand that the second example is contextual (regarding exams), I’ll precise the fact that, Tout s’est bien passé ≠ Tous se sont bien passés (the first one isn’t the singular form of the second one).

Tout s’est bien passé is general, just as when someone asks how your journey in another country went. Tous se sont bien passé in another hand, only works when someone asks you how several things (exams, previous job positions during an interview, etc) went.