I had the same issue today. Didn't used the cam for a week. Was constantly on power connection.
When I tried to start the cam, I just had a black screen. Tried several times, same result. When I checked the forum and found this thread, removing the battery for a few seconds while disconnected from the power, made the cam boot normal again. I do not have any of the power down issues, just to mention that, the cam just refused to boot.
Software version 0.3.2