Yoda has had trouble moving on a couple of occasions due probably to pain associated with his cancer progressing, but he just hopped very slow and stopped and stood alot, didn't seem to be off balance, so the behavior doesn't sound too much like Blitz's.  So hopefully it won't turn out to be cancer-related.  (If it turns out to be though, thus far, painkillers have successfully brought Yoda back to his normal hoppy self).  But it does seem like there are so many possibilities of what it could be that only a vet is going to be able to get you answers.

Try to hold back the freaking out now while you don't know what's going on.  You need to conserve your energy for when you find out what's wrong and what Blitz needs to feel better.  Good luck!  Thinkin' of ya!!

Maybe Blitz hurt his lower back so doesn't want to move. I know my pup Tazzie played too hard once when he was a 4-legger and simply did not move the next day. He seemed extremely depressed so it was scary, but it cleared up in a day or two - at the time we were able to figure out that the problem laid in his back.

At the minimum your vet might be able to provide some pain relief and hopefully some insight into the problem.