a Time War.
Doctor Who Series 3 Episode 12: "The Sound of Drums"

By the nutcases who brought him back in "The End of Time":

Governor: Mrs Saxon. Let me introduce myself. I'm your new Governor. I'm afraid the previous Governor met with something of an accident, which took quite some time to arrange. Miss Trefusis, if you will prepare. You kept your silence well, Mrs Saxon. Your trial was held in secret, with no jury, so no one knows who Harold Saxon was, where he came from, why you killed him. Make her kneel. There are those of us who never lost faith. And in his wisdom, Harold Saxon prepared for this moment. He knew that he might die and he made us ready. Tonight, Mrs. Saxon, he returns. 
Masterinto that new body, or simply aged into it:

William Hughes played the young Master in the flashback in "The Sound of Drums" and "The End of Time"; that was probably his first body
Roger Delgado played the Master during the Third Doctor's era. We don't know if the Master regenerated into the Delgado body, or if William Hughes grew up into Roger Delgado
Peter Pratt and Geoffrey Beevers both played the Master at the end of his regeneration cycle; these are implied to be the same body. Once again, we don't know if there were any intermediate regenerations
Anthony Ainley played the role for the last eight years of Classic Who, but he wasn't a regeneration; his was a body the Master stole
Likewise with Eric Roberts, from the Movie

