Just had a very strange problem with my TivoHD. I could record and play back shows, see all the Tivo menus and the program guide but no live TV on any of the outputs.
Tivo tech said the cable card menue was saying the multi stream card wasn't programed. Tech called Comcast and they said the cable cards werent on my account. Gave the Comcast tech all the info and they repaired the cards to the Tivo and I got my live TV back.
Turns out when I had called Comcast earlier in the day with a billing question they some how lost the cards. What I dont understand was how I could record programs and play them back but not watch them live.