Here are some troubleshooting steps that may help.
- First is to uninstall/reinstall the Monitor driver.
- Open Device Manager and uninstall the Generic PnP Monitor driver.
- Reboot the laptop once uninstalled and it should reinstall the driver automatically.
- After the reboot do test if the brightness will work or not.
- If it will still not work, please do try all these steps on how to adjust brightness that may help.
- And if these steps will still not make the brightness work proceed with uninstalling/reinstalling the graphics card driver.
- Go back to Device Manager and uninstall the graphics card driver under Display Adapters.
- Next is to check Programs and Features then uninstall anything related to the graphics card.
- Once done, download and install the latest driver from the manufacturer site of the GPU. (AMD, Intel or Nvidia)
- Reboot the laptop once the driver has been installed then test it again.