Oh thats not good. You can try this method and see if you can get the Fn key to work. Click Start< Run and type services.msc and hit enter. When the serivices window opens scroll down and locate and highlight the Human Interface Device Access/HID Input Service. Make sure the service is set to Automatic and that it is Started.
If the service is disabled then right click on the service and click Properties. Under Startup Type set it to Automatic and then click the Start button. Click Ok to save the changes and then close the Services window.
I doubt the OSK or a USB keyboard would work, the only chance would be if you knew the keyscan code of the fn Key and remapped it to another key (even then it's unlikely to work). As Arie posted, replacing the keypad is your best option.