When your leaves fall off with a slight bump, it is a definite sign of over-watering. Now that the stem has turned dark and is squishy, it may be difficult to save your plant.
It can be tempting to water succulents frequently, but they are designed for drought conditions. To prevent over-watering in the future, be sure you have your succulents planted in well-draining soil. This will ensure that the water doesn’t pool around the roots, and allows them to soak up what they need and then dry out, as they would in their natural habitat.
If you’ve saved the leaves that fell off, you can try propagating them, though over watered leaves don’t always survive. Let the leaves callous over for several days, and then propagate on well-draining soil.