This happens to me whenever I turn on the wii with a disk still inside. To fix it, I have to remove the disc, turn it off, unplug it for a few minutes, plug it back in, and turn it on without any discs inside. Then I insert a disc and it works... Not sure why it does that, but it's easy to avoid -- all I have to do is avoid turning the console on with a disc inside. Maybe this is what's happening to you...
EDIT: Note that the wii will cache the disc info, so if you boot it up with a disc inside and it's the same one as before you turned the console off, it won't read the channel screen image from the disc again... Maybe this behavior is what is causing the problem...
EDIT: Note that the wii will cache the disc info, so if you boot it up with a disc inside and it's the same one as before you turned the console off, it won't read the channel screen image from the disc again... Maybe this behavior is what is causing the problem...