Thanks for your response. The site was old and neglected, obviously…thus the server move and the attempt to update things.
Simply removing the plugin rendered the site unusable as I described in my initial query.
Fortunately, I discovered that the malfunctions were caused because remaining in the theme template files were the php code requests for ad banner groups. When the plugin was removed or deactivated, those calls caused the site to break. I should have realized this early on, but when I ran into issues updating the program as well, I just assumed it was the program.
The simple fix was to remove all the theme code related to adrotate first. Then, deactivating it worked just fine. I’m rebuilding the site from scratch now. When I get to the point of needing to incorporate ads again, I’ll simply download the new version and start fresh.
Thanks again, sorry to have bothered you. Multiple days of server migration to significantly updated hardware and all the ensuing issues to fix made me look beyond the obvious. LOL.