On my Asus computer, F9 is the key that disables the touchpad. For some reason, it's stopped working. When I press on it, nothing happens. Is there something I may have inadvertently done to deactivate it? If so, how can i reactivate it?

I suppose you have a key named FN on your keyboard. You must hold down that one at the same time you push F9 to make it work.

I hope that was as easy as that to fix your problem, because on your question it sounds like you only pushed F9 without pressing down FN at the same time.

Hope it works for you.

Some (many?) newer notebooks inverted the use of the Fn key with the Function keys.  e.g.  On my Surface 3, the default setting is to use the Fn key to get the <F9> function.  Not using the Fn function would get the touchpad-disabling.

 

Usually there is a way to reverse the default setting on such "modern" machines.  I think I reversed mine in BIOS settings?

Did the unavoidable and asked Mr. G.

(asus notebook function keys not working)

 

One solution that is ASUS specific appears more than once:

http://ccm.net/forum/affich-324412-asus-fn-keys-not-working-for-other-keys

 

Suggest you search for "HControl.exe" (hotkey control) and start it.

 

Did you upgrade to Windows 10 (or what OS do you use)?

A little more complex is missing or non working "ACPI" drivers after upgrade or re-installation.

http://answers.microsoft.com/en-us/windows/forum/windows_10-other_settings/missing-acpi-driver-after-upgrade-to-windows-10/988b8fc7-0855-4f3a-9091-2fec79061d81

That is a problem that I know myself.

Often had trouble to get the brightness keys to work properly.