As the fire in your hearth burns, numerous gases are released and soot begins to accumulate on your chimney walls.
This includes a flammable substance called creosote that can be dangerous to your home. Also, these deposits can begin to obstruct, and eventually completely block, the chimney.
Make sure your home is safe and your chimney free to breathe, and have a qualified chimney sweep specialist come out to your home to thoroughly clean it.