What could be the reason for a TV to be dark while the brightness is a

This is likely an issue with the backlights or the circuit for the backlighting. Not sure what model your tv is but from your description you seem to have backlight issues. The fact you can see an image on your screen by shining a light on it definitely proves you have backlight issues. This is usually caused by either a burnt out backlight or a issue in the backlights circuitry. Replacing the backlights can be a challenge but it can definitely be done. YouTube has plenty of videos which can show you how to replace backlights. Not sure what type of tv you have so I can’t go into much detail here on how to repair it, as I don’t know what type of backlighting it uses. Since you can see the menu while shining the light on your screen you may also see if your firmware is up to date. If not try updating it if possible to see if maybe this is being caused by a firmware issue. If your firmware is up to date you know there is a problem with a burnt out backlight / backlights or there’s an issue in the backlights circuitry.