Thank you for your email. You have done the right thing by putting the bird in the box next to the radiator. The bird may not eat for a day or two which is quite natural as it is in an alien environment and may have enough energy reserves not to eat.
The fact the bird has not been able to stand may be simply shock but it could be pelvis damage. I would recommend you carry on for today keeping the bird warm and quiet, and then tomorrow if there is no visible improvement I would get the bird to a vet and ask them for an opinion.
Suffolk Owl Sanctuary