My guess is that you both [coincidentally] thought that you had a channel highlighted and it was not, but the other one was. I know sometimes it is hard to tell as teh color is just the inverse of what it was. Just a guess.
But I digress, Teddy's original problem sounds awfully suspicious. Almost like a step is being forgotten, or maybe you are even highlighting the wrong channel?
Dang, I don't know.
What I usually do is close everything down, sleep on it, and wake up to find everything normal after a cup of strong coffee.
BTW, just highlight the file and hit "
ctrl+G" to get to the gain window. That way you don't accidentally click in your project wondow and [un]highlight something.