But I does seem to be a touchpad on/off function key. Are you sure the touchpad is on there as well as possibly in the BIOS?
I am using an HP Envy 15z-j100 with Windows 10 and, as of this morning, my touchpad no longer has any functionality and I am restricted to keyboard functions only. The Synaptics Pointing Device is still installed and I even removed, re-downloaded and reinstalled it with no results. When I go into Device Manager, the touchpad is not listed. Similarly, when I go into the Mouse and Touchpad Settings, there is nothing listened/no settings to change. I ran a system restore and it still isn't working. Where do I go from here? Is this an internal hardware problem? It was working fine when I last used my laptop about 36 hours prior.

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic