Lost strength in both upper legs after PRP injection, what could have gone wrong?

It’s very unusual to have loss of strength in the lower extremities post PRP injection. The only thing I can think of is that you may have suffered a condition called Neuropraxia which is mostly a temporary damage to a peripheral nerve due to trauma among other things. I don’t think it’s common post injection, but theoretically if the nerve is hit during the injection process , it can happen. The best thing to do is see a neurologist ASAP. Good luck.