My dog's legs suddenly gave out. What's causing her weakness?

Hello, sorry to hear about Skittles. There are certainly several things that can cause dogs to have problems moving their legs. What you're describing however, seems to be more likely associated with an injury to her back. Most commonly in dogs, they may "slip" a disk which causes compression to their spinal cord. There are other injuries that are not compressive such as an swelling due to autoimmune disease in the spinal cord, a stroke to the spinal cord, or a fibrocartilagenous embolism which is more associated with larger breed dogs.

Again their are other possibilities such as if she sprained a muscle or hurt her knees. Due to the concern for a potential back injury, Skittles should be evaluated by a veterinarian promptly to ensure she does/does not have a back injury. There are 2 options if she does have a back injury, you may elect to treat conservatively with strict rest and at home medications for 4-6 weeks or you may elect to have a neurologist evaluate her. The neurologist may then recommend an MRI and surgery based on the MRI.

I know this is a lot of information to understand/explain so shortly but I would start off by having her evaluated by your veterinarian soon. In the interim, keep her quiet and rested, (no jumping, running, rough playing), make sure she is urinating and defecating, and keep her low to the ground. Best of luck and hope this helps.