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There are two chances for this problem. Either the LCD backlight has failed or you have turned down brightness. To turn up, brightness press the F15 key at top bar on your keyboard to increase brightness. Or press fn and F15 (f14 to reduce brightness)
You will see a sun like logo on these keys. If that didn't work, press F1 or F2 and the brightness or f and f1 and f2
If that didn't work, then please do the following steps
First reset the System Management Controller of your Mac. Follow the steps below;
Shut down the computer.
Unplug the power cord and also disconnect all other devices.
Wait 20 seconds.
Attach the power cord.
Wait five seconds,then press the power button to turn on the computer.
If that didn't work,then reset the PRAM and NVRAM of your Mac. Follow the steps below;
Shut down your Mac
Restart and immediately hold down Command-Option-P-R keys at the same time
Hold the keys down until the computer restarts and you hear the startup sound for two or three times.
Release the keys and let the Mac boot up.
Please let me know if that didn't work, so we can continue troubleshooting