Presumably you are pressing the Fn + The Increase Brightness key (both together) to ensure that the brightness is at maximum
Does the HP “splash screen” show up OK on Startup before the laptop boots into Windows?
With the laptop turned on, try shining a torch at an angle close to the screen to check if you can detect the Windows desktop display at all.
If you can detect the Windows desktop then you have a backlight problem. (see below regarding replacing the LVDS cable or the display assembly)
Just to eliminate a remote possibility of a driver problem, if you have Win 10 installed as the OS, try starting the laptop in safe mode and check if the display is now OK.
If it is OK in safe mode then you may have a driver problem.
Have you checked that the latest graphics drivers are installed?
If it is still not OK in safe mode keep going.
If you cannot detect the windows desktop, try connecting an external monitor to the laptop and check if its’ display is showing OK. Press the Fn + screen display key to toggle between laptop screen, external screen or both screens. (A TV can be used as an external monitor if it has HDMI inputs and you have the appropriate HDMI cable to connect the laptop to the TV).
If the external monitor doesn’t show a display either then there is a problem with the GPU or the motherboard in the laptop.
If the external monitor shows a good display then there is a problem with the laptop’s LVDS cable or the LCD screen.
Here’s a link to the service manual for the laptop, taken from this webpage.
Scroll to p.41 to view the necessary pre-requisite steps and then the procedure to remove the display assembly, this includes the LVDS cable.
Also on p.41 you’ll find the part numbers for the assemblies. If you search online using the appropriate part number only in the search box of your browser, you’ll get results for suppliers of the part.