Curves Vanish

I'm totally new at this. I'm using Unity 4.5 on OSX 10.9.3. I've had it happen several times that after I've set a position and/or rotation curve in the animation window, then watch the animation, I return to the animation window and find that all the curves have vanished and the animation is totally empty. A more experienced Unity user tried to help, but the program did the same thing to him. Can't seem to find anyone else with this problem. Any ideas on what's going on? Thanks!