Explore under /sys/class/backlight or /proc/acpi/video for an ACPI interface for your video card perhaps, sometimes there's a control for backlight power there which might have chosen a stupid value as its default.
Make sure you have the relevant modules modprobed to get those interfaces.
It might be useful to set up sshd, so you can login via something else until you can see your screen properly. An external monitor could also help.
See here:
https://bbs.archlinux.org/viewtopic.php?id=113476