a helicopter can "glide down" just like an airplane. if your engine fails, with the aid of the rotor as you descend you can turn the potential energy of being up in the air into forward motion, and that forward motion will generate lift over your rotor blades. You practice these techniques as part of getting a license.
see the "dead man's curve" which relates to having an engine failure with insufficient height or lateral speed (helicopters shouldn't hover in place unless a good ways up) https://en.wikipedia.org/wiki/Helicopter_height%E2%80%93velo...