Board Thread:Show-Related Questions and Answers/@comment-3456168-20150424014336/@comment-2153169-20150424014742

She did not loose her magic :) In fact, to cast the Shattered Sight, she needs a lot of magic because it is a difficult spell. That's why she can't keep Arendelle frozen. Anna talked about it in "Fall".

If you prefer, it is like she took back the magic she used to freeze Arendelle to reinforce her magic for the Shattered Sight :)