During the Twilight Invasion, she voluntarily surrendered to Zant. This way no one would get killed. But what was the price? People ended up getting killed anyway. They are living inconstant fear and to top if off, she is forced to helplessly watch. And for someone who is but a lonely young matriarch, that had to be really tough for her to do.
Remember when Midna was on the brink of death? Who else was there to save her? Why, Princess Zelda of course. For fudge sakes, she temporarily disappeared after losing her power. That's why the TP reincarnation of Zelda is my favorite; she's a young woman not afraid to make sacrifices for others.
And if you ask me, I think she could have taken on Zant if nobody else was involved. That's just me though.