This is a common bug when using a laptop with an external display. Without knowing your exact laptop model, I don't think I can provide an exact solution, but below is what works for me on one of my testing laptops that has the same issue.
You should be able to manually set the backlight by manipulating the device files in
/sys/class/backlight, and then create a desktop/keyboard shortcut for this so that it's easy to do when you unlock your machine.
ls /sys/class/backlight to see which backlit displays are available. If you only have one, this will probably output something like
To set the backlight, run
echo 0 > /sys/class/backlight/acpi_video0/brightness. This should take effect immediately.
In XFCE, you can right click your desktop to create a launcher/shortcut, or use the settings menu to create a keyboard shortcut.
You'll need root/sudo access to edit the file, so you can set your launcher to
xfce4-terminal -e "echo 0 | sudo tee /sys/class/backlight/acpi_video0/brightness" to make sure that you are prompted for your sudo password.