I have a mesh that I rigged yesterday, and all the bones were visible. Today, they were still visible, but after adding just a couple faces, they now seem invisible. The armature object still exists, same with all the bones, I can even see the XYZ location of the heads and the tails for each bone, but can't find them anywhere. Edit Mode and Pose Mode can be entered, but still, the bones aren't visible. So how can I make the bones viewable again? Below is a MediaFire link.
You're in local view mode. This focuses on the selected objects - in this case the mesh - and hides all other objects.
To toggle the mode use the viewport's menu View > Local View > Toggle Local View or just press the shortcut key Numpad / if you have a numpad. Turn the local view off and you will see the bones again.