-_-, not sure if this would work as I've never had this problem, however I do turn my kb lights off on purpose, to save battery when gaming. You guys can try this:

Go to mce.ini and look for this line:
Modules=display;keypad;vibrator;led;battery;filter-brightness-als;inactivity;alarm;accelerometer;callstate;camer a;homekey;audiorouting

Remove the word 'keypad' to make it look like this:

Modules=display;vibrator;led;battery;filter-brightness-als;inactivity;alarm;accelerometer;callstate;camer a;homekey;audiorouting

(All this does is take off keypad lights.)

Restart the phone and remove the battery. Now turn it on again.

Go back to mce.ini and re-add the word keypad to the line
e.g

Modules=display;keypad;vibrator;led;battery;filter-brightness-als;inactivity;alarm;accelerometer;callstate;camer a;homekey;audiorouting

(This returns keypad lights.)

Turn phone off again, take out battery for about 15 more seconds and switch the n900 on again.

There's no guarrantee it would work though...