Calibri is not a Style but a font or typeface.
Understanding Styles in Microsoft Word
Normal is a Style, an uber-style. It is often the basis for other styles.

I suspect that what you did was select all the text in your document (or a portion of it) and change the font to Cambria Math. You may have applied this change back to the Normal style.

Use the Reveal Formatting pane (Shift+F1) with the problem equation selected. Check the box at the bottom of the pane to Distinguish formatting source.

That should have the information you need to decide what needs to be changed back.