1
Press Fn + Esc to test if that removes the Function Lock on your keyboard.
2
If not, boot your PC into BIOS and check if there is an option in there to toggle between Function Keys and Media/Action Keys
3
If that still does not resolve the problem, if you are using a laptop with an internal keyboard, what is the make and model number of your laptop, if you are using a PC with an external keyboard, what is the make and model number of the keyboard.