She was having alcohol problems. As a professional musician, I can say that moving from a glamorous career to the gutter (she had a serious downfall in the last few years), it really ****s up your understanding of life.
She had over 100 million sales, so her fall was heavy.
I was never a huge fan of her music, although of course everyone's heard her songs and almost certainly seen The Bodyguard with Kevin Costner as well. It still sucks and the world is less for her death (but then again, it is less for the deaths of all those who aren't famous enough to make the news too).