Hogwarts: a History
Rowena Ravenclaw found her way to her deathbed after the death of her husband. Enoch Ravenclaw was seen practicing his magic by a Muggle. The Muggle who witnessed his display approached Enoch from behind and killed him before he could defend himself.
Rowena grieved for months before finally joining her husband in the afterlife, though it is speculated her death was speeded on by Salazar Slytherin.
Slytherin was a Muggle hater, and when he discovered the reason for Enoch Ravenclaw's demise, he approached Rowena with an offer. He would hunt down the Muggle who was responsible for Enoch's death, and he would kill the man himself.
Rowena refused his offer, and Godric Gryffindor and Helga Hufflepuff forcibly removed the Muggle hating wizard from the castle grounds and bade him never return.
From that day on, Salazar Slytherin was never welcomed back into Hogwarts, and only a few short months passed before Rowena departed from this world to move on to the next.
It is said that the wise witch died of heartbreak.