The website Catholic Answers describes that:
The doctrine of the Assumption says that at the end of her life on earth Mary was assumed, body and soul, into heaven, just as Enoch, Elijah, and perhaps others had been before her. It’s also necessary to keep in mind what the Assumption is not. Some people think Catholics believe Mary “ascended” into heaven. That’s not correct. Christ, by his own power, ascended into heaven. Mary was assumed or taken up into heaven by God. She didn’t do it under her own power.
The Church has never formally defined whether she died or not, and the integrity of the doctrine of the Assumption would not be impaired if she did not in fact die, but the almost universal consensus is that she did die. Pope Pius XII, in Munificentissimus Deus (1950), defined that Mary, “after the completion of her earthly life” (note the silence regarding her death), “was assumed body and soul into the glory of heaven.”
Source: Catholic Answers, “Immaculate Conception and Assumption” (San Diego: Catholic Answers) www.catholic.com