This part doesn't make sense: "my installation of Poser 12, which had been working fine, had reverted back to Poser Pro 2012"

I'm not aware of any way that could happen. It seems more likely that both Poser 12 and Poser Pro 2012 are installed on the same computer. Poser Pro 2012 would have the problem RedPhantom describes. It's possible Poser 12 would stop working, for example because the registration information was never entered and it was actually in the free trial period, but it wouldn't uninstall itself, and it certainly wouldn't uninstall itself and install an older version in its place.

If Poser 12 did stop working, and it's not simply asking for a registration code or an internet connection, that sounds like something to ask Poser support about. https://support.posersoftware.com/