So sorry to hear that your succulents are in distress.There are a few common reasons for leaves to fall off.

It is getting cooler and if your succulents are getting a chill, from cold tap water or proximity to the window, they may drop some leaves. Your window sill can be quite a bit colder than the rest of the room so move them away a bit if they are close and it is cold outside. Use tepid water on your plants. Sudden heat from a heat vent or hot draft has the same effect.

Soil that is too dry during the growing season or too wet in the winter can also cause leaves to fall. When the plant is in active growth, allow the soil to dry completely between waterings but do not expose the plant to prolonged drought. In the low-light conditions of winter (October through February), water only as often as is necessary to prevent the leaves from puckering.

Your succulents should be in a container with a drain hole, in fast draining soil that does not have fertilizer added to it. Succulents mostly grow in low-nutrition habitats and too much fertilizer can have a toxic effect.

Courtesy of NYBG Plant Information Service