I’m not sure if it’s just the lighting, but the base- bottom pad still in the pot looks a bit yellow. If that’s the case, it’s probably been overwatered or had difficulty processing water from the roots for whatever reason. Dropping healthy pads when the plant is not healthy or even worse, near death, is a reproductive measure.

Pull the remaining pad up and check for root rot, mushiness/softness in the pad, or any other indications of infection and dispose of it if you find any. Check the one next to it as well. Make sure your skull pot - super cute, btw! - has sufficient drainage and very little water retention going on in the soil. Amend the drainage and soil as needed.

As for the fallen pad, if it is firm and healthy you can definitely leave it on its side on top of soil and it will begin rooting. If you’d like it to stand upright, wait about 2-3 weeks to ensure it is fully healed before propping it upright with the joint about 1” into the soil. Compact the soil around it well and use stakes as needed for the first few weeks of rooting. These root very quickly with the first watering after healed over, so they’re pretty cool plants for propping.