Add Media button in new post stopped working

When drafting a new post, Add Media button no longer works. A successful post with media was added FOUR days ago, then TWO days ago the Add Media function stopped working suddenly. Also, on the same Post Edit page, the “Visual” tab, the “Shortcode” button, and the Add Tag feature toward the bottom do not work. All buttons are present, but nothing happens upon clicking.

Other sudden symptoms: Jetpack works but not the Site Stats (no error, just continues to load but never does).

I already deactivated all pluggins, but issue still is present. Not a browser issue either.

I use a custom theme. I am running WP 4.0, which was recently updated SEVEN days ago.

This is weird because of the perceived randomness; everything was working fine and these functions suddenly stopped.

I’m very appreciative of any assistance.