Try starting it up in "Safe Mode" and see if it will let you change the brightness in there. If it will, then there is something on the tablet that is restricting it. Bad app, virus, malware, etc. If, however, it won't change even in "Safe Mode" then you are probably talking hardware issue.
To get into "Safe Mode" please do the following.
1. First turn off the tablet.
2. Next press and hold "Power" until you see the manufacturers logo on the screen, then release the power button.
3. "Quickly" press "Volume Down" and hold it for few seconds until you see the message "Safe mode: ON" or something similar.
4. If you wanted disable it, simply repeat the steps above, and you should see "Safe Mode: OFF" rather than on.