I went out to the dish and wiggled it in every direction with no change in signal level (on 61.5), so it's not an alignment issue. I suspect a bad LNB, but there's something bothering me, which is that a very few transponders on 61.5 work fine. (The one with my HD locals in particular.)
When I go to the system info screen, I get a yellow box for 61.5, and when I hit the Details button, it says that I've lost signal dozens of times in the past two days. I don't know if the report process only goes back two days, or whether the problem only started then, because I watch a total of ONE channel on the affected transponders with any regularity, and the last time I watched that one prior to last night was last weekend.
The affected transponders that carry channels I subscribe to (but most of them are SD, so I never watch them -- except for VH1 Classic, which doesn't exist in HD) are 6, 12, and 16. As I said, most of the other transponders are weak or gone, and a very few are strong enough to watch.