Kid Buu upon defeting Goku and the rest of the z fighters he continues as he wanted.. destroying his universe upon which he gets very bored and as a result very angry and so rips a dementional barrier ...

Both Buu and Janemba may have been destroyed thanks to the combined forces of Goku and Vegeta, as the red demon was blown away by Gogeta in the movie and Buu was turned to dust thanks to a Spirit Bomb ...