I'm having this problem now on a new Roamio Pro, but the solutions mentioned in this thread don't seem to work for me. The first thing I tried was rebooting, but when the TiVo comes back up, the video is still stuck in the preview window!
Then I tried changing the output resolution as mentioned in this thread, but that still didn't get me out of this state (I tried a number of different resolutions, my TV's preferred resolution being one of them).
Another thing that's slightly different is that the original poster mentioned that the remote buttons didn't work when in this state - in my case, they do.
Does anyone know of any other possible solutions to this problem?