In any case, it was a blessing in disguise that it didn't turn into anything serious.
Literal
Whatever-the-case, serious-matter [to-に] not-reached [nominalizer-の] [topic-は], misfortune-inside [possessive-の] fortune was.
Two set expressions in one sentence. 何はともあれ ('whatever the case, in any case') is an idiom used to refocus on the most important point. 不幸中の幸い ('fortune within misfortune') is the standard Japanese equivalent of 'a blessing in disguise' or 'silver lining.' 大事にいたる ('to come to a serious matter') uses いたる (至る), the literary verb for 'to reach.'