OK so we can rule out the power saving features and backlight switch.
You might want to verify that it isn't a hardware issue. How old is your netbook? A bad backlight may cause a display to become dim.
Maybe try popping in a live DVD of Trisquel or some other distro. If it's a software issue and it was working before, then the problem shouldn't show up when you start up the live DVD. If the problem is still there, then I would assume that your backlight needs replacement and a trip to the repair shop will fix that.
