Ways to Become Wheelchair Bound?

My FMC is a wheelchair-bound princess, and I don't know why. She can move her hips, but she can't walk. It's never really come up in what I was writing, but I figure I probably ought to explain it (as she's a super-magical mage, and in-story it's a big surprise for one of her kind to have this particular problem). I am trying to stay away from car crashes, though, for various reasons. So. Ideas?

Well, spinal cord injury is a big one, as you know, but that doesn't have to happen by violent accident. There's birth defects, like spina bifida, and illnesses, like polio or muscular dystrophy.

And there are a lot of folks in wheelchairs who aren't paraplegic per se, but who can't reliably stand or walk long enough to use anything else. There was a fellow in the movie "Unbreakable" like that - his bones were so brittle as to make even a fall from standing-height catastrophic.

So if it helps to narrow down your options, you might like to think about how whatever-it-is may affect your lady's personality. For example, if it's something like muscular dystrophy or multiple sclerosis, which will either possibly or almost-certainly mean she's got less time to live than most ordinary people, she might have an extra iron-willed seize-the-day attitude to life. If she was born how she is, then she'll be plenty used to it and have a good idea to handle the stares and occasional questions. If it was something that only just happened a couple of years back, then she might still be working through her feelings about it, and have some considerable frustration or anger or resentment to get past.

I think once you decide on the above, you will have an easier time in finding a specific condition/situation to match.

Perhaps she had a spell go wrong, which disabled her legs. Since it was a spell that caused the problem, the usual 'heal' spells can't fix things.

Another option would be for her to be physically able to walk, but not want to walk because it would cause problems. (If she walks, it breaks the spell that keeps a very destructive demon at bay.)

She traded her ability to walk for the ability to use magic.

Trampled by a horse? Maybe she was bucked from a horse and her legs were trampled, bones broken, and never properly healed? Or she could be like that kid in The Secret Garden whose wheelchair bound-ness is purely psychosomatic.

stoopid horse scared of a little snake reared up just as she passed under the stoopid low treebranch

In the manga Mushishi, there was a character who had a sort of demon inside her that covered her leg and made her unable to move it. Or you could go the Sealed Evil In A Can route like Generalist said, and have the lower half of her body be the cage for something. Or it could be the result of the 'versal equivalent of a demon deal, expanding on what DeathOfScythes said: She got something she wanted, but every time she uses it, the paralysis spreads further up her body. Or because she's so powerful, her magic could interfere with anyone else's who tries to heal her, and by the time she learned healing it was too late to fix her legs.

Traumatic Brain Injury. Having someone beat you within an inch of your life. Suicide attempts. Poisoning.

Well I know my friend James ended up in a wheelchair because he went cliff diving in a place that was too shallow and he broke his back. So he can't feel anything south of his tailbone. Well through surgery he is working on walking again. But he went cliff diving, and that's something that can be worked into a fantasy novel