Thanks Fernando! We found the Unhide All button under the Edit Geometry panel, just where you said it would be. We got our vertices back.
The confusing part is we right-clicked to hide the vertices, but when we right-clicked to Unhide All they don't come back.
How do you know so much about 3ds Max? Have you taken any classes in it, or are you self-taught? How many years have you been using the software?