Cool, thanks everyone for all the suggestions.
The guide data all came back after I did a "Redownload Guide" on each source. Perhaps the guide was "downloaded" while the Internet was out, of course with no data received, and that meant it wouldn't try again for those time slots. Redownload fixed it.
Thanks for the Plex info. I set mine up for open LAN access so that should be OK now. Hadn't thought of that before, and of course it's hard to change if you can't login because the Internet is down.
While I was there, I added the Channels folders for Movies and TV to Plex as new libraries - "Channels TV" and "Channels Movies". I also previously added my Plex folders (containing content from my DVDs etc.) to Channels as a local source. So now all recordings are available either in Plex or Channels. That may be useful since the Plex app is available on my Smart TVs where the Channels app is not. So even if the various sticks don't work, I can hopefully get at recordings through the Plex app. No way I see though to get live TV that way.
I should be much better prepared for the next Internet outage. Thanks again.
I still remember when TVs had dials for the few channels you could get, and rabbit ears or the antenna on the roof needed constant adjusting. Things have really changed, but gotten a lot more complicated.