I have had this same exact problem happen twice in the past. A couple of years ago when our first digital CBS station came on line, they had a few problems. One of those problems was that sometimes they were not broadcasting what they thought they were broadcasting. My 6000 was getting a signal and locking onto that, but there was no picture info to decode, or it could not decode what it was getting, so I got a blank screen telling me I was getting 93% signal from a digital station. The box would lock up and you could do nothing but wait for it to work itself out, which may take 15 minutes, or unplug it. There were probably only about 15 or twenty of us digital users in the area at the time, so sometimes no one would call the station to complain, thinking that someone else had already done so. Even when you would call, initial glances at their computer would tell them they were broadcasting, it just wouldn't tell them that it was undecodable. The only way it got fixed was through our mail list or list serve, or whatever you want to call it. We all started complaining about the problem on the list serve, and the assistant engineer, (who was a member, and who had been telling us all along that their station was definately broadcasting and there really was no problem on their end) finally figured out there was an issue he had to fix. Long story short, it got fixed. It happened again once after that.
I know this is a long story, but one way to determine if it's the station's problem is if the box re-maps the channel differently that it used to. Our CBS station is broadcasting digital on channel 17, but the analog station is 12, so the 6000 (using PSIP I suppose) remaps the 17 to show up in the guide as channel 12, in yellow, designated as a digital channel. When our problem occured, suddenly there would no longer be a channel 12 digital in the guide, but there would be a channel 17 digital. If you switched to that channel, the box would lock up.
My advice would be to try and locate other digital users in your area and ask if they have lost that channel's picture as well.