GOT IT BACK. Did everything here a dozen times. Called Verizon. We reset box. Then we reset the Fios remote (press "back" and "last" together till red light, punch in 3 numbers, 983 I think.) Still wasn't working. Accidentally switched input from HDMI to "TV" and lost my TV for a long while. Kept bouncing through all the inputs multiple times, with both Fios remote and TV remote. Minutes later TV came back. And with it, volume control. VZ rep didn't know why, but said maybe it was the switch from HDMI to "antenna", as he called it. Whatever it was, it worked for me. Nothing else here did. Good luck! 1 800 VERIZON. Ask for Steve.
Sounds like the input switching reset the HDMI pairing between the box and your TV. Next time try powering off the TV and the box. Remove the HDMI cable between them, turn both back on for 2 or 3 minutes, plug HDMI cable back in. That will usually reset the HDMI pairing and cause a new handshake between the two devices.