Same problems with my Z500 series, there is a way.

You need to boot into BIOS/EFI, then at the keyboard press the brightness/dim key accordingly, then restart normally.

Your screen should be brighter now after restart, then you also need to prevent Windows from dimming the screen automatically by going to the Power Option & disable the auto dim function.

My Z500 BIOS mode is just a small button beside the AC jack, yours I'm not sure. Maybe you need to refer to your manual on this.