My Lenovo ThinkPad SL 400 display is too dark and cant control with the brightness control buttons (Fn + Home or End keys). Once Laptop battery power was really down and this started happening after that. You can get the screen if an external monitor is connected Thanks for your post

go to start-->control panel--->display--->adjust brightness

i need to know what operating system you use this will work for only Vista and 7 and 8 and 8.1

hey Robe, try these:
from Lenovo:
http://support.lenovo.com/en_US/downloads/default.page?
Hotkey Utility:
http://download.lenovo.com/ibmdl/pub/pc/pccbbs/mobiles/8jvu39ww.exe
Access IBM button (for Windows XP):
http://download.lenovo.com/ibmdl/pub/pc/pccbbs/mobiles/1rth52us.exe

Have you checked that the NumLK key is not on?

Maybe a crumb under the Fn key. Give it a stab and see. Otherwise go to start-control panel-monitor-adjust brightness. Hope this helps.

Try using Thinkvantage Application to adjust as follows:

ThinkVantage Control for Display Brightness

• 4 Click the "Start Menu," then click "All Programs" and locate the "Access IBM" program group.
  
• 5 Click "Thinkpad Configuration" and click "LCD" to open the brightness control dialog. Select the brightness from the drop-down menu and click "OK."
  
• 6 Click the red "Exit" button in the upper-right corner of the Configuration utility to close it.
  

Read more: How to Adjust the Brightness on an IBM ThinkPad Display ' eHow.com http://www.ehow.com/how_5818403_adjust-brightness-ibm-thinkpad-display.html#ixzz277iQ4g48

You should try to determine why your Fn keys are not working... just to make certain, the Fn+Home key will increase brightness and Fn+End will decrease.

You can try the blue IBM-Access Key. In the Configure tab, you can manage the display settings.

If you cannot see your display because it is too dark, try attaching an external monitor via the vga connector.

You can reduce the brightness by pressing Fn Key and END Key together (press Fn key first and Press END key to reduce the brigtness as desired.Fn will be written in blue color on key.Fn stands for FUNCTION)

To increase brightness you can press Fn key and HOME key.

The screen will automatically dim slightly when it is on battery power, this is to conserve battery power. For maximum brightness connect the power adapter into the laptop.
On the Thinkpad laptops you can adjust the brightness by pressing and holding down the blue Fn key plus the Home key to brighten or the End key to dim the screen.

ok bottom left hand side of keyboard there is a key with fn on it hold this down along with other blue keys that have like little suns on them this is your brightness keys

Have you increase the brightnes by Fn key and brighntess key on above of keypad please set it first fn>+brightness key
Also please check wheather the deaault settings are in agp card settings in display properties

Regards
Balisonu79