after upgrade to 12.04 login to desktop is not working on lenovo t420s

Try the boot option:

nomodeset

May help

Thanks for the reply. Didn't help though. It booted with a crappy resolution like in recovery mode but the behavior is the same. Any body have a clue what could be causing this??

Idea, rm or rename ~/.Xauthority and ~/.drmc, reboot.
There is no fingerprint gui in the way?
http://darkblue.homeip.net/fingerprint/Forum/rss.php

I had the same problem, I simply stopped all the lightdm processes with 'ps ax | grep lightdm' and 'kill', then started the service back up with 'service lightdm start'. That worked for me anyway, hope it helps. No idea what was causing it though.

Okay, on next boot I had the same problem but stopping the processes didn't do anything. Quick dirty fix for me then was just to 'apt-get install lightdm --reinstall' that 'fixed' it. I'll monitor the log files and anything else I think could be related to try and figure out what is going on.

Ok, so here's an update and "probable" solution. I basically experienced the same as this guy:
http://askubuntu.com/questions/126275/cant-login-after-using-ubuntu-tweak/127663#127663
Only difference is that most ppl on that thread seem to be able to log in every 2- 3 reboots. With me I must have rebooted 25 times but nothing.
Eventually however, I was able to log in for whatever reason, I had changed nothing I just kept trying. Once logged in I did a variation of the solution of cbourbou in the askubuntu thread: I installed ubuntu-tweak. I then chose a background image for the login screen (any image). I didn't expect it to work but after rebooting I was able to log in every time now. I've used it for a few days now and have not had the issue since.

I honestly don't know what caused this, I'm not a linux newbie but definitely not an expert either. I do feel like issues like these are what keep ubuntu from becoming a bit more mainstream and keep it in that <1% market share for desktop users. An upgrade to a new version and the user can't log in, and the solution is to reboot 50 times and hopefully get a successful login? That's a bit embarrassing when I tell me windows friends that. I love ubuntu but PLEASE make it so I can put it on my mom's computer! :)

Thanks for all your help guys

I forgot to mention I also cleaned up some old packages and configs using ubuntu-tweak.

Just a thought for the future: whenever I upgrade, I expect that I may have problems. I now prefer to do a clean install at every version upgrade. It's quick, easy and hassle free.

Ever try upgrading a Windows version? And then spending the next umpteen days trying to get your hardware working again? That's why Windows folk go out and get a new computer rather than attempt a version upgrade. And when they buy one with Windows 8 they will be flocking to use a diffferent OS won't they?

@Barry: I suppose I should do that from now on but "quick, easy and hassle free"? Really? I don't see how having to re-install all my third-party software, settings and custom scripts is NOT a hassle or quick. Besides, I always felt like a clean install is a windows-people problem :)

A login screen which hangs on "logging in" doesn't seem like something coming from an "unclean" installation. If it is indeed from changing the background image then it just seems like a weird bug.

But you are probably right in that it's the least problematic route. Thanks for your thoughts ;)

Sorry! However, a word of advice. Don't do a clean install until you are satisfied that 12.04 doesn't have a problem with your hardware. Boot the live DVD and choose 'Try Ubuntu'. Make sure all your hardware works OK. There have been a few problems with certain nvidia graphics controllers and a very few with other hardware. Mostly it's down to a handful of graphics controllers.