Why is my Mac screen not brightening, and how do I fix it?

Your Mac probably isn't broken. First restart it. Then in System Settings open Displays and turn off auto-brightness and True Tone. Make sure the slider isn't stuck at the bottom, and check that Low Power Mode is off. Those are the usual culprits when the keys feel dead. Apple Silicon Macs have no SMC to reset; a restart or Safe Mode does the same job. The separate case is a maxed slider that still looks dim. On a MacBook Pro 14"/16" or Pro Display XDR, that's the everyday cap macOS sets, not your panel's limit. MacBrightness opens the brighter range Apple holds in reserve (about 1,600 nits) on the same keys, free to try, $5 to keep.