Not sure what the posted code has to do with the question. You should make use of [code=cpp][/code] tags when posting C++ code, for nicer embedding and highlighting.
For your simple situation, you can introduce a boolean that tracks whether the game has ended.
Then you put an if block around the updating functionality for the ball and the paddle (and the AI).
And finally you put an if block after the paddle and ball drawing, to render the win/lose text.
As long as the game isn't finished, you update the paddle, ball and AI.
Once it is finished you draw the text over everything else.
A more general (and scalable) solution is to use some sort of a state machine.