Comment 6 for bug 505176

I had the same problem: after the upgrade when logging the monitor turns off and on several times but without show login screen. I solved downgrading xserver-xorg-video-ati and xserver-xorg-video-radeon with versions downloaded from:

https://launchpad.net/ubuntu/+archive/primary/+sourcepub/944871/+listing-archive-extra

I have a Sapphire Radeon 4850:

01:00.0 VGA compatible controller: ATI Technologies Inc RV770 [Radeon HD 4850]

Next are some logs.

P.S. in xorg log I read

(II) LoadModule: "ati"
(WW) Warning, couldn't open module ati
(II) UnloadModule: "ati"
(EE) Failed to load module "ati" (module does not exist, 0)