Expresses relief that something bad did not happen or that a favorable outcome occurred. Often used at the beginning of a sentence.
Fortunately, it didn't turn into a major accident.
Luckily, he was safe.
Fortunately, it didn't turn into a serious accident.
Fortunately, all the passengers were unharmed.
幸いにも and 幸いなことに are near-identical set phrases meaning 'fortunately', with 幸いにも slightly more literary and 幸いなことに more explicitly clause-initial.
Derived from the adjective 幸い (さいわい, 'fortunate') with the adverbial particle にも, which adds a sense of 'in a fortunate manner' or 'as luck would have it'.