ask the manufacturer.
We might have had some suggestions but apparently when you posted the Note in red text just above where you entered your subject title was missing so you neglected to bother telling us any particulars such as manufacturer and model. That note that is normally there is as follows:
Note: If you're asking for technical help, please be sure to include all your system info, including operating system, model number, and any other specifics related to the problem. Also please exercise your best judgment when posting in the forums--revealing personal information such as your e-mail address, telephone number, and address is not recommended.
A generic answer is to check your user manual and see if there is some system setting that allows it to be turned on or off and failing that to maybe CAREFULLY pop the key loose and clean under it.
I turned on my computer and out of nowhere I discovered the Fn key is permanently on, I have to push it to get the keys to function like they would were it not pushed. *** has happened and how do I fix it?