Reset the SMC (System Management Controller):

Shut down your MacBook Pro
Hold down the Shift + Control + Option keys and the Power button
Release all the keys and the power button at the same time
Turn on your MacBook Pro

Reset the NVRAM (Non-Volatile Random-Access Memory):

Shut down your MacBook Pro
Hold down Command + Option + P + R
Press the power button to turn on your MacBook Pro
Continue holding the keys until you hear the startup sound twice

If the backlight issue persists, check the brightness settings:

Go to System Preferences > Displays
Adjust the brightness to see if it fixes the issue

If the above steps don't work, it's possible that the backlight itself or the cable connecting it to the logic board is damaged and needs to be replaced by a professional.

Note: Before trying any of these steps, make sure your MacBook Pro is running the latest version of macOS and all updates are installed.