Given a sudden onset, EC is probably not the cause. It does cause hind leg paralysis as an early symptom. It is possible that the paralysis has been developing gradually and you couldn't see it until it became disabling. Almost every bunny has been exposed to EC at one point in their life and it lays dormant until some time at which it becomes an active infection. Very little is known about how EC progresses from dormancy to active infection. A stress to the immune system can be a cause.
However, this parasite can be very difficult to treat. It can sometimes be treated with Panacur, but often a stronger antiparasitic is needed, and many of these stronger drugs are not approved for use in rabbits, so you will need a vet that's willing to use a drug off-label in a very expensive treatment. I hope our resident EC expert, Randy, who has treated many bunnies with EC, will step in here as I cannot seem to remember the name of the drug. Marquis, maybe? I believe it is approved for use in horses.
My best guess is that it is a bone or nerve injury. The sudden onset and the degeneration of tissue make me think this. Some bone injuries do heal on their own, or are too small to be seen on x-ray. Nerve damage due to trauma also cannot be seen on an x-ray.
Working with less than $250, there isn't a lot you can do. The test for EC isn't 100% conclusive and is quite expensive (probably at least $80). Also, although I obviously am not a vet and cannot diagnose through the internet, i think it's more likely to be an injury due to trauma. If you do not amputate the leg, she may get an infected wound on it (no feeling=doesn't know that she got an injury=doesn't take care of it, similar to how diabetics lose limbs to infected wounds they couldn't feel due to diabetic neuropathy), and that could take her life. She probably needs to be separated from the chickens to prevent further damage and she needs to have her leg amputated. just my :twocents