So this just happened to me today. Here are two ways to try and find it and both worked for me pretty easily. First is to check the left hand side of your window. Under the preview window you should have a tab, within that tab you should see Graphs with at least EvenGraph listed and then Animation Graphs with AnimGraph listed under it. Click the arrow next to AnimGraph then double click the AnimGraph that drops down.
The second way is to go to the top of the window and click the menu item Window. In that menu click Find Results which opens a box at the bottom of the window. In the search bar type AnimGraph and press Enter. Double click AnimGraph and it should open up.
If neither of these options work for you there might be something else wrong.