Michonne's death isn't a permanent one in the comic books. She's killed off in a bonus ending to Issue #75 though many people thought it was the legitimate ending to the comic, even forcing the creators to state it was just a bit of fun. After the main canon of the story, the bonus ending sees Rick wake up in an infirmary (not like the beginning of the series) and when he eventually finds Michonne again, she explains that Rick disappeared for weeks and in that time aliens invaded the planet and used the walkers to eliminate civilisation, while the remaining humans were tasked to harvest water, a currency on the alien world. Soon after this, the Governor returns as a cyborg with a robotic arm and crushes Michonne's skull. It certainly was just a non-canon bonus ending at the end of the issue, but imagine if any of that made its way into the show? Lunacy...