In short, Orthodoxy does not believe in the Immaculate Conception. We believe that Mary was conceived in the ordinary way, without any special freedom from original sin based on "prevenient grace", and with the effects of the ancestral sin.
The rub is that Orthodoxy believes that the effects of the ancestral sin are death and corruption -- full stop. We do not believe that a man is worthy of condemnation to hell just by virtue of being born.
We believe that the Theotokos had a miraculous birth, similar to that of St. John the Baptist (her parents were elderly and childless -- a familiar Biblical story.) We believe that this birth happened for a reason, just as did the miraculous births of Sampson, Samuel, and St. John the Baptist. But the morally guiltless life of the Theotokos was not the result of some special grace given her at conception. She had the same ascetic tools at her disposal that all the rest of us do. She is a true role model of obedience and sanctity for us.
Being born with the ancestral sin, she grew old and died. Her salvation is completely dependent on the work of Christ, since without Christ conquering death, she would be held captive by Hades just as were all the Righteous of the OT prior to the Resurrection.