From the manual:
But there’s no button any longer in NU12. All I see is this:
(… needless to say that there are no 32 bit plug-ins at all.)
Consequently I can’t re-activate a suddenly black-listed plug-in that used to work until yesterday evening. 8-/
You can delete / edit the file:
%AppData%\Steinberg\Nuendo 12_64\Vst2xPlugin Blacklist Nuendo.xml
You didn’t mention your OS (above link is for Windows obv, you can copy and paste this link to your file browser).
Try reinstalling the plugin in question, and re scan by quitting and reopening Nuendo.