My mouse wheel is suddenly rotating around an object instead of zooming in. What hotkey did I accidentally press, and how do I get it to zoom again?
That generally happens if you press Ctrl + Alt + Scroll Wheel or Shift + Alt + Scroll Wheel by default.
The fact that it is doing it by default may mean that you either changed the keyboard shortcuts in Blender preferences or you have a stuck key in your keyboard.
As far as I know there is no keyboard shortcut to permanently set this behavior by default