While on an away mission in 2366 to study the homeworld of the Koinonians, who destroyed themselves long ago in a lengthy war, she was killed by a remaining Koinonian bomb. Lieutenant Worf, who commanded the away team, felt personally responsible for her death, despite Captain Jean-Luc Picard telling him it was an accident.
The other sentient species on the planet was a race of pure energy beings. They had witnessed the Koinonians destroy themselves in the generations-long war. Sensing the pain of now-orphaned Jeremy Aster, the energy beings created a replica of Marla Aster to raise him. Counselor Deanna Troi felt he had to get over the death of his mother, and this would never let him do that- coupled with the fact that the sheer scale of what the replica would have needed to provide for Jeremy in order to give him a complete life was impossible-, so Jeremy asked the replica to leave. (TNG: "The Bonding")