Many succulent problems show visible signs before the plant is lost. Sunburn, leaf drop, translucent leaves, stretching, black rot, and soft stems each point to different causes.
Look at recent changes first: light, watering, weather, repotting, pests, or poor airflow. A plant problem is usually connected to a condition, not bad luck.
Respond by correcting the cause. Move slowly when changing light or watering, and isolate plants if disease or pests may spread.