Hit upon the notion of her reading it in an old book.
But this needs some justification. Yes, it's a classic fantasy trope of ancient magic being better, but it does need some.
The obvious explanation, used rarely, is resource depletion. Larry Niven ran it to the logical conclusion in The Magic Goes Away. But that's obviously unworkable for magic she can practice, even if it would let her happen on a magic artifact.
But another one would be catastrophe. The Mound Builders build -- I hope this is not too much of a surprise -- enormous mounds in the Mississippi Valley. Then the Spaniards came, not enough to conquer, but enough to bear disease. The resulting population crash not only broke down society, it left some descendants of the Mound Builders unaware that the mounds had been built by human beings. Note that this is on a grand scale. Merely fall of Rome doesn't cut it.
(And the fun thing is that this backstory is just plot device. To get her where I need her.)