Hi there
As a vet, I agree with several of the possibilities mentioned.
"Vestibular syndrome" is anywhere from an infection in the middle ear, on the way into, and including in the brain, where something affects balance, making dogs very travel sick to the point where they can be physically sick or just not able to get up, it is often sudden onset, but they pick up quickly, and medications can make it go away.
Strokes are also common in older dogs, and whether it is a clot or a bleed in the brain, often it will happen suddenly, but then the brain learns to compensate, the damage corrects itself over time and they can go on to do OK.
Another possibility is the arthritis/denenerative changes in the spinal area. The only way to be sure what's going on with that sort of area is an MRI or a CT scan. However, I had a collie in a while ago, who had been a bit stiff behind for a while, and was getting progressively worse, weak in the back legs, and when I saw her she was completely unable to stand. The vet school saw her, and decided to try treatment before stepping up to MRI scan, and on a course of steroids and a drug called gabapentin that works really well for pain in nerves, she is doing amazingly well, and is now on low doses of both drugs, and is back to being able to chase a ball gently on occasion! So don't despair, and if it is something that happens again, do mention this to your own vet, they may or may not think this is a suitable course for your collie.
Hope she stays well

Suzanne