I'm sorry you fell! Hope your knee is feeling better by now.
Maybe it would interest you to know that it's actually totally normal for dogs to ignore their owners falling over. There was even a scientific study about it! People (who had signed up for the study) would walk their dogs through a park and then pretend to have a heart attack -- they'd groan, clutch their chests, and then collapse and drop the leashes. The idea was to see if any of the dogs would go to a nearby "stranger" and seek help. But actually, all the dogs sort of stood around for a bit, then wandered off to go sniff bushes. Well, except for one dog, who went over to the "stranger" and hopped into her lap for a cuddle!
The researchers decided that maybe the set-up was just too subtle for dogs...maybe dogs just don't understand heart attacks? So they designed a second study, where owners would walk their dogs through a building and then pretend to get trapped under a falling bookshelf (it had been rigged to fall so they weren't hurt, and it seemed a lot less subtle than a heart attack!). And all the dogs stood around wagging, and some of them pawed or whined at their owner, but mostly they just acted confused before wandering off to explore the room. Which doesn't mean they weren't concerned, just that the set-up didn't really produce any consistent "seeking help" or "providing comfort" sort of behaviors.
It's not that dogs don't care. It's just that dogs didn't really evolve to offer comfort to a friend who has fallen over...not just your dog, but most dogs! Your dog probably cares about you a lot, but she didn't have the tools to know what to do in that situation. If there's something specific you want her to do when you lie on the ground, you can train her for it. Just decide ahead of time what, specifically, you want her to do, and give her lots of chances to practice with lots of tasty rewards. Like all training, it should be fun!
(The study, which is certainly open to critique: Macpherson, K., & W.A. Roberts (2006). "Do Dogs (Canis familiaris) Seek Help in an Emergency?" Journal of Comparative Psychology 120 (2): 113-19.)