Under Site Settings>Navigation, I incorrectly checked the Show subsites box under Current Navigation. This enabled a link to all subsites in my left pane (which I did not want). Instead of going back to the Navigation settings (which probably would have avoided this issue), I clicked Edit Links from my main site and was able to hide the subsites (but not remove them, which I was fine with).
The only problem was that after I clicked Save, SharePoint deleted all of the other current navigation links that we had to our team site libraries and lists. Can someone please explain why it removed those links? Nothing was changed accept to Show subsites in that pane and nothing was deleted.