The answer to 'deck turning brown' is that the most common causes are weathering, mold/mildew growth, and improper staining or sealing. Symptoms include a dull, faded appearance and potential cracking or peeling of the wood. Solutions typically involve cleaning, sanding, and re-staining or sealing the deck to protect it from the elements.
If the deck is only lightly discolored, you may be able to handle the cleaning and re-staining yourself. However, for more significant damage or if you're unsure of the best products and techniques to use, it's best to call in a professional landscaping or deck maintenance service. They can properly assess the condition of your deck and recommend the right treatment to restore it.