ok, we will undo any system corruption that has occurred. There is no default setting, but if the brightness via the instructions I gave you had no effect, it is a corruption issue.
Go to the Windows 8 Start Screen and type restore point. When the search results appear click on the Settings category.
Now click on the option labeled Create a restore point and you will be brought to the System Protection tab of the System Properties control panel.
Click on the System Restore button and you will be presented with the main screen for System Restore. Now click on the Next button and you will be shown a list of available restore points that you can restore.
Select the restore point you wish to restore by left-clicking on the entry once. Use a date when this was working, so before yesterday
Then click on the Next button at the restore point selection screen. You will now be at a screen asking if you are sure you wish to perform the restore.
Click on the Finish button. System Restore will once again ask if you are sure you wish to continue. If you are sure, please click on the Yes button.
System Restore will now reboot your computer and begin the restore process. Please be patient as this can take quite some time.
Then tell me if the brightness is functioning correctly.
I can also do these steps for you, and have made a request via Additional Services. This is optional, and if you choose to do this, I can, with your permission, remote into your computer while you watch and do all these steps (and deal with any problems that arise during them) for you. Or if you do not choose to take this service, we can continue here. This is no problem at all. Just let me know please.
Else let me know the results please