First, try pressing Fn + F9 key combination and check if it is working. If not, try to press Fn key and then F5. If it still doesn't work, turn the laptop off, remove the battery and all the cables and press and hold the power button 15 seconds. After that, plug in the cables, insert the battery, open the computer and press F2 when the Toshiba window appears. Next, press F9 to reset the configuration to default and then F10 to save the changes. Is you still have issues, navigate to Toshiba Support page, identify your computer model, select your operating system and download and install the latest drivers available.