Obviously the point is moot now (glad you got your system running), but I was more suspecting something like maybe a damaged root block on your DH1: or some other structural error which led the system to not accept the volume, but left all your data (probably) intact on it.
I would probably have started with a Check, and if it turned up anything, take a deep breath and use Repair and/or Recover.
I don't think the file system is likely to have suddenly changed by itself, so my guess would have been that it was FFS all along.
Just my two øre (which - like these suggestions - are worthless in Denmark today
).
Best regards,
Niels