If the media keys on the keyboard aren't working properly, please use Window's Search tool to look for and open "Services.msc." Scroll through and open "Human Interface Device Access." Make sure the Startup Type is set to "Automatic" and below press the Start and Stop buttons to stop and restart the service. After you have done this, hit APPLY and OK.
Models: KB115, KB505U, KB506, KB506W, MGK Series, MGK-L80 Series