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Unfortunately, these plants have a tendency to do that. They can die all of a sudden without any warning. There is no way to prevent this. This article will give you more information: https://www.gardeningknowhow.com/ornamental/shrubs/winter-daphne/daphne-plant-types.htm

Daphne is rather picky about its growing conditions. More importantly, it settles in for a couple of years before it flowers. A good root system is the shrub's priority. More info at this webpage: https://www.gardeningknowhow.com/ornamental/shrubs/winter-daphne/daphne-not-blooming.htm

20 years is the average lifespan for these plants. Sometimes it will be less. There is nothing that you can do to keep it alive for longer. You will have to replant.

This article will help: https://www.gardeningknowhow.com/ornamental/shrubs/winter-daphne/daphne-plant-types.htm

The more sun the better for these, although in hotter climates, need some shade to perform well. They will do well in partial shade no matter what, but full shade will inhibit their growth, and it will be very slow. This article will help: https://www.gardeningknowhow.com/ornamental/shrubs/winter-daphne/pruning-winter-daphne.htm

Unfortunately, it could be either. You should only water when the top two inches of soil are almost completely dry. This article will give you an idea of what they need as far as care goes: https://www.gardeningknowhow.com/ornamental/shrubs/winter-daphne/daphne-plant-types.htm

Yellowing leaves can indicate watering issues, disease or pests.
This article will help you review care.
https://www.gardeningknowhow.com/ornamental/shrubs/winter-daphne/pruning-winter-daphne.htm