Have you try using the F-keys at the top to turn the brightness all the way up? Turn it on and let it sit for 5 minuets to make sure it has gone to the desktop if it is working. Then use the F-keys marked for the brightness. Wither or not you have to hold the FN key at the same time depends on your settings. If that doesn't work try resetting the PMU
. If that doesn't help, then try resetting the PRAM. Do this by turning it on, then imeadeatlly hold the command, option, p, and r keys all at the same time, you should hear it do the startup bong again, keep holding them down until you hear at least 3 bongs. If still nothing, then turn it on and hold a flashlight up to the apple on the cover, and you should see if there is anything on the screen. If not, the LCD is dead, or not getting any signal. If you do, then the backlights are bad. Either way it has to go back to Apple for repair.