She travels back to Hyrule

When Midna shattered the mirror of twilight she had thought she was making the righteous and noble decision to separate the two worlds for the betterment of all. Yet, in the end in only took about a week for Midna to begin regretting her decision.

She couldn’t help it. In her time in Hyrule she had grown fond of Link and of the other princess, Zelda; and though she was among her people again she felt loneliness clawing away at her. She loathed to admit it, but she missed Link. But she had her duty, the realms had to remain separate and besides, the mirror was already gone. It’s not like there was any other way of traveling to Hyrule.

A few weeks later, long after giving up on the prospect of staying true to her decision, Midna discovered another way of traveling to Hyrule.