According to the biography of the Buddha,he was a Mahaprusha(great being) named Svetaketu.Tushita Heaven(home of the contented Gods) was the name of the realm he dwells before taking his last birth on earth as Gautama Buddha.There is no more rebirth for a Buddha.Before leaving the Tushita realm to take birth on earth,he designated Maitreya to take his place there.Maitreya will come to Earth as the next Buddha, instead of him coming back again.Krishna was a past life of Sariputra,a chief disciple of the Buddha.He has not attained Enlightenment during that life as Krishna.Therefore,he came back to be reborn during the life of the Buddha and reached the first stage of Enlightenment after encountering an enlightened disciple of the Buddha
He reached full Arhatship or full awakening after become ordained in the Buddha's sangha.
