(covers information from several alternate timelines)
A crash landing was an emergency maneuver that was performed when there was insufficient time to land the ship or craft properly, or when the starship lacked the ability to land. Crash landings were often caused when the vessel was sent hurtling into a planet's atmosphere either too fast or with no control by the helmsman.
In 2371, the USS Enterprise-D was forced to crash its saucer on Veridian III after it was caught in the shock wave of the exploding stardrive section. The helmsman had no control over the crash, but the descent of the ship was leveled somewhat by rerouting power to the lateral thrusters. (Star Trek Generations)
In an alternate timeline, USS Voyager was forced to crash on a class L planet after being thrown out of slipstream. The vessel would have broken up if they did not attempt to land, but the vessel came in too fast and crashed violently on the planet's surface, destroying one of the nacelles and killing all hands but Harry Kim and Commander Chakotay, who were on the Delta Flyer. This timeline was averted by the future Kim when he sent course corrections to Seven of Nine, which collapsed the slipstream, saving the crew. (VOY: "Timeless")