There are several reason that your brightness key does not work. In most common HP model, FN key is important to control certain function by pressing it with another specific key on your keyboard. This includes to change brightness, volume, WIFI, Bluetooth, Screen Lock, and some other functions.
Note : f your current screen are dim, click on start, type "Change Screen Brightness" on the search box, and press enter. The use the slider to change your screen brightness. The solution below is a workaround to permanently restore you brightness key function.
1. Install Quick Launch:
Quick Launch (available for download on HP website) is the software that controls the hotkey function. Try to install this first before proceed. However the brightness failure is not mainly form Quick Launch button but this is a good step to start with.
2. Check Your Monitor Driver.
Open device manager and look at your monitor, if it shows standardPNP Monitor, right click on it and select uninstall.
Then right click again (anywhere you like) and select scan for hardware change.
Windows should automatically install GenericPNP monitor and now you can change the brightness setting. (Note: Reported by HP G6 Model user).
IF your Monitor already show GenericPNP. Right click on it and select Enable, if there are no "Enable" option and you only see disable, your monitor is not the problem.
3. HP 15 Notebook and HP DM4Notebook - Update Graphic Driver:
One of the solution especially for HP 15 and HP DM4 is to update the graphic drivers. you can obtain the graphic driver from the HP website or from the manufacturer website. For DM4, you need to install the Latest catalayst from AMD website. For HP15 (Intel model) go to Intel website and let it detect the latest driver for your notebook. (for Intel graphic version 8.2 is recommended).
4. REFRESH NOTEBOOK SETTING
I don't know what we should call this process, but it is similar to resetting your BIOS.
(This won't erase your data)
1. Press Power Button until your notebook shutdown (Force Shutdown).
2. Remove the battery for 4 minutes and put it back.
3. Turn on the laptop until you are logged in.
4. Restart normally using the start - restart.
5. Uninstall "IntelliType 8.2"
If you installed this software from Intel Website, Uninstall it.
6. Sometimes there is an odd bug.
Try to plug and unplug the power chord ( you will see the LED dim) and try to adjust the brightness.
7. If you have Microsoft Curve Keyboard..
Plug in Your Microsoft Curve Keyboard, make sure you are connected to the Internet, let it search, download and install the drivers. Try the brightness key after installation finish.
8. Update your BIOS.
Go to HP software and download, download the latest BIOS and update your BIOS. This solution tested and work for some HP model such as HP 2000 xxxx series.
Some notes (will add more update later):
1. For HP 4540s : Install HP Hotkey Support File Name : SP56876