RESOLVED BUT POSTING AS A FYI
This is very strange, but over a period of time our P4 started losing reception on channels - I didn't notice until my husband complained - as he's the one that watches offbeat channels.
Yesterday it got a lot worse and a tons of channels we're saying "no signal" so we contacted the cable support people.
Our setup -
BrightHouse standard cable
We were watching something on our AppleTV via network (not Tivo), then something on YouTube via Tivo - then that video froze. At that point we were not receiving ANY cable channels.
I reset the Tuning Adapter a couple of times - no help there
Figuring it was fault with the cable company I went to watch a recorded Tivo show - and nothing would play.
The Tivo had to be unplugged.
That cleared it up!
The advanced level tech from Brighthouse said that Tivos need to be shut down every week or so to clear them out. I don't know about that - but in this case, for whatever reason, ours died a slow temporary death until it was power cycled.