This has happened to me several times while playing so far, and I just quit the game and re-started a new one.
I am wondering if it is extreme lag however, because another time when I thought it was going to happen to me again, I ended up doing something for a few minutes, and when I cam back to the game, it was working again.
I have had several instances of super lag, especially when combining stacks of units, and then even a game crash when I sent a stack of units onto a ship.
So I wonder if it is all related. I'll keep an eye out and try to save it to send in if I can find it.