Why would a cat's fur suddenly become matted?

Our cat, Sadie, age 8, has long fur, and up until the past year has NEVER had one tangle. She is pretty much an indoor cat, but does go out for an hour or so several times per week. Now she has an entire body full of matted fur ! Any ideas as to what would cause this ? How to prevent ? How to get these mats out ...they are almost matted to her skin !