Yes, tragically, at least one astronaut has been lost in space.
Vladimir Komarov was the first astronaut to die during a space mission. His fatal accident occurred on April 23, 1967, during the Soyuz 1 mission. While not "lost" in the sense of drifting away, he died in space due to a parachute failure during reentry. The spacecraft crashed upon landing.
Although Komarov died during reentry and not while "lost" drifting in space, his death is a significant example of the risks inherent in space travel.
