Something similar happened with Shane a year ago, though it wasn't quite as overnight as yours sounds. Neither x-rays nor veterinary hands, including those of a rehab vet, could figure it out. We finally did an MRI of the spine and saw that he had a healed fracture of the pelvis from who-knows-when. As we had dieted him, his thigh muscles got smaller, and it turns out it was those muscles keeping everything in place. Shane wasn't in apparent pain either, but he would have intermittent limps and slowness on walks. After at-home massages, acupuncture/chiropractic, strategic exercise, and Adequan injections at home, his muscles are back and he does much, much better with walking.
Things like this can only be found with MRI's. We were lucky, ours was "on sale" for $1000, as a special to welcome the hospital's new neurologist...and drum up business, no doubt! But it also came in handy later when some UTI/kidney issues arose, to be able to look at that and see that the kidneys looked structurally good. So I'm saying that you can get to see a lot of things that are useful to know, even beyond what you're looking for.
ETA: I think Racindog is right about incurring damage to the spine, with big consequences, if this is allowed to persist. Sorry.