Bougainvilleas are tropical plants but can return from the roots after a light freeze. Some people therefore overwinter the plants in a cold location such as a garage where the temperature will not drop below about 30 degrees. Keep it moist but not soggy wet during the winter months.
Your plant probably dropped leaves due to a period of dry or soggy wet soil. The first is usually not a problem and the plant should releaf soon when it gets moisture again. Soggy soil however can lead to root rot. Allow the plant's soil to dry well between waterings to prevent this. Another common cause of leaf drop is a sudden decrease in light intensity. This is common when moving a plant indoors. Try to place it in as bright and warm a location as possible if you want to keep it growing over winters.
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