I may have pushed a key unawares because suddenly I can't see my selection highlights. If I click a face/vert/edge I can manipulate it but it's not highlighted orange, can't see it when I press "a" either... where can I turn this back on?
You have turned off the Show Overlays toggle on the user interface. Click it! Some pair of circles in a little square button. Overlays On, they're Blue, Overlays Off, they're gray as I see they are in your little gif I'm unsure what keystroke activates or deactivates them but the mapping for keystrokes is easy to look up under Edit > Preferences