Are you using SendKeys?

This is what I have used in the past on the "On Key Down" event, but it seems to be inconsistent..  You can use sendkeys and the numlock option, but if the numlock is on, then that shuts it off....

    ' The Keycode value represents the key that
    ' triggered the event.
'   Select Case keycode
    
        ' Check for the PAGE UP and PAGE DOWN keys.
  '      Case 33, 34

        ' Cancel the keystroke.
   '     keycode = 0
  '  End Select

Are you sure it's Access that is "turning off the num lock key"  Just asking because my system does that often and not just with Access.  Started after some Windows upgrade (i have 3 computers. 2 with Windows 11, and the other with Windows 10). The machines that have "full keyboards" have the problem happen.

It would be nice if when a form is loaded to check and see if the NumLock is "On" and if not ask if you would like it on...

Yes this is a huge pain.  I have some clients blood pressure going up due to this.  The code I found above actually works, but as noted is inconsistent...  There should just be a windows setting that turns it on.  Sendkeys "{numloc}",true works, but there has to be a setting to tell it that it is still on as the above will shut it off...

What EXACTLY are you doing that turns the Num Lock off?

I'm not sure....  Something is causing it and there is a lot of VBA that could be the culprit...