Est-ce que tout ce que l’on se promet a pu se perdre dans un désert ? Ne reste-t-il que des poussières une fois que l’amour est absent.
Could everything we hoped for have been lost in a desert? Nothing but dust remains once the love is lost.
With que, introducing a full clause in the perfect aspect.
Une fois qu’elle l’aura vu, elle le comprendra. ― Once she’s seen it, she'll understand it.
Introducing a reduced passive-voice clause consisting only of a subject plus a past participle phrase.
Une fois le déjeuner fini, vous pourrez remonter. ― Once dinner is finished, you can go up.
Introducing a reduced perfect-aspect clause consisting only of a subject plus a past participle phrase (provided that the verb is one that forms the perfect aspect with être).
Une fois les professeurs arrivés, les élèves expliquèrent tout. ― Once the teachers had arrived, the students explained everything.
Introducing a reduced clause in either of the above senses, consisting only a past participle phrase, its subject being implied.
Une fois sortie de cette maison, elle se sentit libérée.
Once she was out of that house, she felt liberated.