It’s possible that the reason your Mac keyboard backlight won’t work is that you’ve accidentally dimmed it to the lowest Ievel. The steps for confirming and undoing this might differ depending on which Mac model and macOS you’re using, but none of them are difficult.
On some models, you’ll have to open the Apple menu > System Preferences/Settings > Keyboard, then drag the “Keyboard brightness” slider to the right until the backlight is illuminated to a level you’re satisfied with.
Per instructions from Apple, if your Mac has keyboard function keys, typical of 2015 models and earlier, you can increase the keyboard brightness key by pressing the F6 key. F5 will turn the brightness back down if you want to do that. On MacBook models with touch bars, simply tap on the left arrow on the Control Strip on your touch bar, then tap the “increase keyboard brightness” button. It’s the one that looks like half of a rising sun. And if you have a 2020 MacBook model or newer, you can turn up your keyboard backlight by clicking the Control Center at the top right corner of your menu bar, then select the Keyboard Brightness tab and drag the slider to the right.