Tune to the unavable channels, go to menu, system information, press info. Look at channel data, SNR level, it should be good. Also look at OOB data, SNR level, it should be good too. If either are poor, check ALL coax connections and splitters. If they check out, you may have a bad box. Happened to me a while back, 2 or 3 channels wouldn't show. Call or email tech to get a replacement box. I'm assuming you are using a box. If not rescan channels with your TV tuner and note if it's an older TV, some channels are now sent as mpeg4 , which many older TVs can't decode.
I have 3 channels that have displayed Unavailable for the last few months. Before that these had channels had no problems. Also get various channels that pixilate occasionly (only non HD channels).
It means you have a bad RF signal at the box. It really could be a lot of things from a bad tuner in the box, cabling, splitters, or the Verizon ONT. Like previously stated make sure everything is tight and try to get a replacement box. If still not working you will need to schedule a tech visit.