Hi Jeric, if Lions' suggestion doesn't do the trick - you may want to reset the user interface - make sure that if you have any addons that you don't want to lose, that you instead move the WTF, Cache, and Interface folders out to the desktop, having also renamed them (like 'WTFx', Cachex, Interfacex). The UI folders are then restored to their default setting. That should do the trick - you can then replace the new folders with the old ones (be sure to name them back to their originals) and see if the issue persists. Hope that helps you!
Sounds to me like you might have accidentally unbound your zoom. Check your keybindings settings and see if anything is bound to wheel down, and what zoom out is set to. Try rebinding it and see if it works.
If you use any addons like Clique that affect keyclicks and binds, check those as well.