Hi,
Please do try these troubleshooting steps that may help.
- First is to try and uninstall/reinstall the graphics card driver.
- Go to Device Manager and uninstall the graphics driver.
- Next is to open Programs and Features then uninstall anything related to the graphics card.
- Once uninstalled, download and install the latest driver.
- Reboot the laptop once the latest driver has been installed then test it again.
- Test it again after the reboot and see if the brightness will work or not.
- If it will still not work, do try adjusting the brightness manually inside Power Options.
- And if that will still not work do run System Restore.
- Here's how:
http/windows.microsoft.com/en-us/windows-8/restore-refresh-reset-pc
- And lastly, if all these will not work I would suggest reinstalling Windows.