I figured it out courtesy of tech support.
My laptop is a Panasonic FZ-55. There is a “feature” which was designed for the Air Force. It lowers the brightness to 1 nit. It is engaged by pressing Fn8. I had accidentally engaged this setting.
The feature is for pilots who are flying in total darkness and need to see the computer screen. The setting overides all other brightness settings until unengaged. You can also disable this setting in the BIOS.