Don't worry, your Might is safe and sound! 5v5 ranked battles do not share the same Might as 3v3 ranked battles. Before we introduced 5v5 ranked battles, your 3v3 Might was displayed on the main menu, but this has now been changed to 5v5 Might and will start at 0.
You can still view your 3v3 Might in the Rank menu. To learn more about Might and how it works, read this article.