It's quite an old Mac.
Try trashing your InDesign preferences.
Yep, it's old, but it's what we've got, unfortunately. Would I do this by deleting a particular file in my Adobe InDesign CC 2015 folder in Applications, or can I do this natively from InDesign? Thanks Derek!
Close and relaunch InDesign, and IMMEDIATELY hold down Ctrl + Alt + Shift (Windows) or Cmd + Ctrl + Opt + Shift (Mac), and respond Yes to the dialog asking if you really want to replace the preferences.
(If you're are not asked the question then you haven't held down the keys quickly enough!)
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We were able to delete cache files (Mac OS X) for InDesign (without having to reinstall InDesign) to restore functionality:
Restarting InDesign (without having to restart the computer) brought InDesign back.