Macbook Trackpad Click Failing For Literally No Reason

Check the Trackpad System Preferences to make sure that "Tap to Click" is still selected.

Option Boot the laptop by holding down the Option key immediately after the startup chime. See if the Trackpad click works to select the "Macintosh HD" volume to boot from.

Does the Trackpad have some movement when you press it to click? Do you hear a physical clicking sound?

Edit: Do you notice if the palmrest, trackpad, or the Bottom Case are deforming due to the battery swelling? Does the laptop sit flat on the table without any wobble?