Comment # 5 on bug 1112638 from Stefan Dirsch

Hmm. Nothing obvious I could spot in the X logfile. 

Possibly a regression in nouveau X driver. I suggest to uninstall
xf86-video-nouveau package (nouveau X driver) and restart X (or reboot). If
this doesn't help, also uninstall package Mesa-dri-nouveau (again restart X or
reboot). If this doesn't help either, start your system with kernel boot option
'nomodeset' (fallback to fbdev X driver, i.e. unaccelerated 2D operations).

Or displaymanager doesn't come up for different reasons.

Please provide this information:

   update-alternatives --display default-displaymanager

If it's not set yet to xdm, I suggest to try that one first.

  update-alternatives --config default-displaymanager

and/or another simple one like lightdm. The restart displaymanager via

  rcxdm