Polar Bears do not hibernate and remain active throughout the year
The only time polar bears do something resembling hibernation is when a female polar bear makes a den in which to give birth and then take care of her cubs. Other than this polar bears remain active throughout the whole year.
During the winter however polar bears enter a state which is known as walking hibernation. During this period of walking hibernation polar bears are able to maintain their body temperature while reducing their metabolic rate and recycling proteins. They usually enter this state in the winter because this is the time when food is scarce for the polar bears.
When a female polar bear is going to have kids she builds a den in the ice which she stays in to have her cubs and tend to them after their birth. Polar bear mothers often go eight months without food while living in the den she has built with her cubs. Female polar bears build these dens by scraping a tunnel in the snow and then digging out two chambers. The female polar bear does not however enter a state of pure hibernation as a higher body temperature is required for nursing her cubs.
Hibernation is very common and widespread among animals, who commonly enter hibernation during the winter when it is colder and harder to find food. Often animals will eat lots during the warmer periods to build up their fat reserves for hibernating and surviving during the colder months. While polar bears do not enter a proper state of hibernation other species of bear do. However bear hibernation is believed to be very different from the hibernation of other animals. This is mainly because during bear hibernation, bears do do not experience as big a drop in internal temperature as other animals.
You can read more about hibernation here
Also check out polar bear endangered