Lenovo T420 laptop, I cannot find the hash key

Lenovo has many of their product manuals on their support page: https://pcsupport.lenovo.com/us/en/ . The Thinkpad T420 documentation is here: https://pcsupport.lenovo.com/us/en/products/laptops-and-netbooks/thinkpad-t-series-laptops/thinkpad-t420/documentation?linkTrack=Caps%3ABody_SearchProduct&searchType=3&keyWordSearch=T420%20Laptop%20(ThinkPad) . The current link to user guide for the T420 (in English) is https://download.lenovo.com/pccbbs/mobiles_pdf/t420_and_t420i_ug_en.pdf . Other documentation is on the site, including the documentation for the included software and a hardware maintenance guide.

I hope this helps.

Cindy Wells

Can you plug a monitor into the external Video connector, and do you see a display there? If you see a display on the external monitor, then the LCD is most likely bad.

Try to restart the computer. Lenovo computers can be glitchy a lot, and restarting seems to help. If it's a touch screen, the computer can be "confused" on which mode of input you're using.

There should be a function key to adjust the brightness at the top of your keyboard my laptop is F2 or F3 so at the bottom of the keyboard is a key label FN The Function key looks like a sun with an arrow if you are lucky to have it labelled.
If this does not fix problem you may have a faulty laptop . There is a circuit called an inverter in your laptop that converts DC to a higher voltage AC to run the fluro backlight behind your laptop's LCD screen. If this inverter goes faulty you could experience the symptoms you are seeing. This would require a replacement of the inverter circuit board in your laptop by a repair service centre.
http://www.youtube.com/watch?v=fSCJzA0EBo0
http://www.youtube.com/watch?v=2Yi6GNFcP3w
http://ask.metafilter.com/92210/Backlight-or-Inverter-How-can-you-tell

What version of windows are you running?Check out tutorials on youtube for you Windows version.

Download Center ,here is driver for you :)

http://www.google.com.ua/search?q=Lenovo+THINKPAD+T420&oq=Lenovo+THINKPAD+T420&sugexp=chrome,mod=8&sourceid=chrome&ie=UTF-8#hl=ru&tbo=d&sclient=psy-ab&q=Lenovo+THINKPAD+T420+recovery&oq=Lenovo+THINKPAD+T420+recovery&gs_l=serp.3..0l2.1216.3253.0.3714.9.5.0.4.4.0.223.701.0j4j1.5.0...0.0...1c.1.14pecXA4UK8&pbx=1&bav=on.2,or.r_gc.r_pw.r_qf.&fp=30f8933bc1ffddc8&bpcl=38625945&biw=1366&bih=643

For T420 model Lenovo doesn't call it "User Manual" instead calls it "Hardware Maintenance Manual".
Here's a link to that manual:
http://download.lenovo.com/ibmdl/pub/pc/pccbbs/mobiles_pdf/0a60007.pdf