The camera view bounding box is not showing up when I go to camera view mode '0'. No box guide-lines, no passepartout (although I checked to see if it was set to medium strength, which it is). The bounding box enclosure does however appear if I click "Render" (not render mode in viewport, the 'official' render button).It's been a very long time since I worked on my blender project and I have no idea what happened. Is there something obvious I'm forgetting? Please help!
A 'Bounding box' in Blender is the 3d space defined by an imaginary box with sides parallel to the axis of the object's local space:
They can be used for simplifying complex scenes or simplifying various simulations or also may help visualizing the space an object occupies and so on.
Clearly you are using the term incorrectly.
What you are talking about is the camera's 'passepartout' or overlay around the camera's view. It can be switched on and off in the camera properties in the Properties Editor under Display section:
You may not be able to see the camera or the overlay if you are looking through it and are zoomed in too much, and also have locked the camera to view in the side panel of a 3d View (the n panel):
You would need to unlock it and zoom out in that case.