This is an excellent example of "theatre of the absurd", filmed as a "film noir" suspense thriller. "Theatre of the absurd" posits that the universe is a random entity where nothing is likely to happen as expected. I believe Camus and other French writers of his era were the principle exponents of this view of life. Well, here we have certainly an example.
A businessman (Julien Tavernier, played by Maurice Ronet), in love with his boss's wife (Florence Carala, played by Jeanne Moreau), plans a murder to look like suicide. This involves using a grappling hook and rope to enable him to climb up a floor without using the elevator. So far, so good. The boss is duly murdered, our "hero" climbs back down, takes the elevator down, and then realises he has left the rope dangling. Well, back up in the elevator, but alas, the building custodian shuts off the power, stopping the elevator between floors. Frantic (also an alternate title along with "Elevator to the Gallows"), our man tries to escape, but he is truly trapped.
Meanwhile, at street level, things are not going well for the man's inamorata. She is wandering around wondering what happened to her man, and is "vagged" by the gendarmes for not having her ID papers. There's more. A pair of wandering teenage lovers (Louis and Veronique, played by Georges Poujouly and Yori Bertin), decide to steal the man's car. They know who he is, and freely use his name while committing a few other crimes of their own. Such as stealing a Mercedes 300SL, and murdering the owner and his wife in the process. When the man finally does get out of the elevator after the power is restored in the morning, he discovers that.... But that would be telling, and you will want to find out for yourself. In any case, I have told enough to show that for the protagonist of this tale, the universe is truly a place not to be trusted at all!