The next step is to troubleshoot the video subsystem.
First, look for an LCD cutoff switch or button on the laptop (some older models have this feature). The switch must be on for the LCD panel to work.
Use the laptop’s brightness function keys to increase the screen’s brightness.
If you added a graphics card to the laptop, make sure that the motherboard’s on-board GPU is disabled in BIOS/UEFI.
Use a port on the laptop to connect to an external monitor. Use the display function keys on the laptop to toggle between the original monitor and the external monitor. If the external monitor works, then use in to go to Device Manager and update the video drivers. If this works, great, if not then replace the inverter, and then the LCD panel of the original monitor.
If both the external monitor and the original monitor don’t work, then there is probably more than one problem with your system.