That's actually what happened to me as well right after I posted all that. Chatted with Comcast, they said it could have been from a power surge at some point, blahblahblahblah. I went from all channels (200+) to 8. To 0. within hours.
Chatting got most of them back, they had to "send a signal" to my cable box. Realized I was still missing some, now 2 hours later, after the "wait 45 minutes so the signals can get there etc..." I'm back to about only 12 channels and losing some here and there.
Maybe it's a local problem?
eta: I just went into the cable card status thing and it says "In order to start cable service for this device please contact your cable provider. 800-266-2278" This has gotta be on comcasts end...