Here is what the Cathechism of the Catholic Church has to say about the Assumption:
966 "Finally the Immaculate Virgin, preserved free from all stain of original sin, when the course of her earthly life was finished, was taken up body and soul into heavenly glory, and exalted by the Lord as Queen over all things, so that she might be the more fully conformed to her Son, the Lord of lords and conqueror of sin and death." The Assumption of the Blessed Virgin is a singular participation in her Son's Resurrection and an anticipation of the resurrection of other Christians:
In giving birth you kept your virginity; in your Dormition you did not leave the world, O Mother of God, but were joined to the source of Life. You conceived the living God and, by your prayers, will deliver our souls from death.
As noted by others, there is wiggle room for her death in the statement that she was assumed "when the course of her earthly life was finished". There are some difficulties with either scenario, to wit:
1. If she did die she would have endured the wages of sin, which is death. That means, of course, that she was not sinless. Christ, who did die, endured the wages of sin, not of His own sin, but for the sins of the world. If He carried the penalty (death) of the sins of the world, then that would leave the penalty to be endured, for everyone else in a physical sense of the word and for unbelievers in the spiritual sense of death. No Christian believes that Mary endured spiritual death, but if she endured physical death her death was either the result of her own sin inherited as a result of the Fall or it may be, as a few have asserted, that it was in relation to her role as co-redemptix with Christ in atoning for the sins of the world.
2. If Mary's death was redemptive in nature, then Christ's death and atonement were incomplete and insufficient in and of themselves for the redemption of mankind.
3. If Mary did not experience physical death, then it would be proof positive of her immaculate conception and sinless life. If she did die, however, it would mean that sin had entered her life, probably at the point of conception, meaning that she was not immaculately conceived, which denies the Catholic dogma of the Immaculate Conception.
Let's chew on these three for a while.