Okay so I was watching Batman Beyond: Return of the Joker. And it seems the original Batman and Joker were away somewhere for sometime before returning to Gotham of Terry McGinnis. I didn't get to finish the movie. But I wanted to ask where they were? I don't really know much about the batman beyond universe.
He was retired and helped Terry McGinnis in his training. It will be full of spoilers how Terry become new Batman but you can watch Batman Beyond (TV series) to know about it.
He was dead as told by original Batman in the film himself and we already know who is the new joker.
Note: Also remember Batman Beyond is part of DC animated universe.