Too much water is indeed a possibility but is easy to fix by simply cutting back. It is also possible that the plant has been attacked by some type of stem boring weevil. Check around the base for signs of soft and/or rotting stem tissue (especially near the base). If you find such contact your County Extension Office about what to do. Basically it will be difficult to save that stem but you may be able to reroot it and side shoots may soon emerge which could be protected from future attack by periodic treatments.
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