Hi Yvonne,
I’m sorry to hear about your pigeon. There can be many causes for this and the only way to diagnose the issue is with an Avian Vet. If these are outside aviaries, no matter how clean you keep everything, the birds are still exposed to diseases and parasites from wild birds, as well as anything carried by biting insects, rodents and other wildlife. Since she had been laying eggs, a severe calcium deficiency or an injury from the egg laying are both something to look at. Assuming she has a mate, this could have been a breeding accident. A head injury can also cause this. If something spooked her, she could have hit the top or side of the cage hard enough to injure herself, even if there is no apparent injury. I have had birds completely paralyzed by head injuries that eventually recovered. Abdominal tumors are also very common in pigeons, but generally any paralysis is gradual. Without the right diagnosis and treatment, she isn’t likely to recover on her own. I hope your vet is able to help her.
Take care,
Brenda