According to J. K. Rowling's official website:

"One of the four famous Founders of Hogwarts School of Witchcraft and Wizardry, Rowena Ravenclaw was the most brilliant witch of her time, though legend has it that a broken heart - cause unknown - contributed to her early demise."

Without giving away any DH Spoilers, I believe that the "broken heart" was explained in Chapter 31 of Book 7 by... someone who told their "story" to Harry. According to what we are told, I assume that the person telling the story was responsible. I would go into more detail, but I fear the ever-present mods...

In terms of a Rowena ship, I can see Rowena/Salazar easier than Rowena/Godric. Slytherin, after all, was known for being cunning and sly, qualities that would blend well with the wit and intelligence of Ravenclaw.

Assuming that Rowena loved Salazar, her heart could also have been broken when Salazar left the school for good.

Hope that helped!