Not much to say about that. I've seen many similar questions but didn't solve the problem. Animation plays fine. It was visible earlier and I didn't do practically anything except switching views and trying some menu options.
You have "pushed down" the action and created a strip. Select the strip and press the Tab key to tweak it. As the manual says:
The contents of Action strips can be edited, but you must be in Tweak Mode to do so. The keyframes of the action can then be edited in the Dope Sheet.
EDIT Also see this question: Can action strip converted back to keyframes?