Depending on where you're located, this IS the time for winter tetany/grass tetany. I've seen several cows through the diagnostic lab here in the last couple of weeks with low Mg levels. Usually more of a threat in a cow nursing a calf under a couple of months of age, but can occur in heifers/steers, especially on wheat/rye pasture and the like.
Beyond that, there's any number of possibilities, but I would suspect that if your veterinarian did a good physical examination he/she would have found pelvic or long bone fractures - but possibly not a spinal fracture.
Any chance she 'did the splits' on some slick ice?
If you're gonna try to pull her through, she needs to be supported/lifted frequently - or, if you don't have that capability, you need to bed her down with a THICK layer of hay/straw underneath her and flip her at least 4 times a day so she's not spending all her time on one side. Many folks don't realize that cows/horses are so big/heavy that if they're down for more than 8 hours or so at a time, that their sheer mass, resting on the muscles of the 'down' side starts to cause muscle damage and necrosis due to impaired blood flow. Even if they have a nerve problem(like obturator nerve damage/swelling from a difficult calving) that will improve over a few days' time, if the muscles are severely damaged by staying on one side, they won't be able to get up, even if the swelling goes down or the nerve heals/repairs.