Writing a book is not easy, and the problem may have nothing to do with feeling uncreative, stressed or tired. The reason may be that you simply don't have enough information. Spending more time researching your topic may be just the answer - and this goes for fiction writers as well as nonfiction.
Good fiction and nonfiction are both based on knowledge. For fiction writers there are some excellent books available on a wide variety of subjects such as plot and character development - books describing lifestyles in various centuries for historical fiction, or forensic techniques for mystery writers.
Today it is often easy to do research without ever leaving home. Just head to the internet and check it out. You local library can be a great second source. But remember, nothing can replace what academics call orignial research: actually getting on the telephone and talking to a human being or going to see something you are actually writing about. You will discover that orginal research will enable you to add depth to your writing.
Research can be anything from a few minutes on the internet verifying a quotation, to a few hours or even days talking to sources or checking facts in libraries. And it can be very rewarding. If you discover that you are truly blocked and can't move forward with your writing, take some time to think about what you may be missing. You might find that a little bit of research is just what you need to get you started again.