There is no 'substitute' if that's what you're asking.
How come it's not working? What happened before the problem?
Notes:
* FN key may not work if the OS has been reinstalled, or the driver has become corrupt. To solve this, simply download and install the appropriate drivers from your laptop manufacturer's website.
* Physical keyboard key broken - Replacement laptop keyboards may be for sale from the manufacturer. If the laptop is still under warranty, they will replace the keyboard for free.