To get back from the credits scene press "X" (the B button on the NES gamepad)
To advance further in what I assume you mean to be the initial opening cutscene, press "Z" (or "Y" if you're on a QWERTZ keyboard, this is the A button on the NES gamepad). Additionally you can skip the text forward in the opening cutscene by pressing "K" (Start on the NES Gamepad).