Well it seems like an ok question for the 'Mac basics and help' forum to me.
First step is to check that you haven't inadvertantly lowered the brightness - check System Prefs, Display and play with the brightness setting. Or use the F1 and F2 keys on your keyboard.
Second step if that doesn't work is to reboot - probably won't do anything but its free and only takes a minute. Then repair permissions - again, unlikely to do anything but still free!
Thirdly, compare it with someone elses PB. Could be that the easiest way to do this is to go to an Apple Store, in which case it should then be easy to do...
The final thing, which is to ask Apple.
You could try a search of this forum, but you'll probably just find loads of threads on the latest PB screen probs and dead pixels.