Answer: The mushrooms at the base of the tree are likely the "fruiting bodies" of a fungus that is feeding on dead wood, so once you see the mushrooms there isn't anything you can do... there are usually a sign that the tree is dead or dying, rather than the cause of death. It's impossible for me to know what caused your tree to die, so I can't say if your other one is OK or not. If it's a treasured tree, you could call in a professional arborist to evaluate its health.
Sorry I can't be of more help!
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