This sounds eerily similar to what happened to my BC a few years ago. She'd have been 6 ish? Maybe 7? No sudden crash, but just didn't have strength in her back end to walk. I can't honestly remember how it resolved in detail, only that the vet didn't find anything initially, next step would have been MRI but I didn't have the money and she seemed to be improving. I presume we ruled out anything like infection or some sort of bug bite reaction, or.....
My personal best guess is a minor stroke, only because of the loss of movement and some interesting personality changes afterwards.
Nothing similar has happened before or since, she's in fine health now, and I've not heard of anything similar in other dogs until now.
Always a good idea to check diet and exercise anyway. Hope she keeps getting better!
Out of interest, where in the country was she bred? Mine's from Wales. Highly unlikely to be a family thing but you never know!