After Muon I can't reboot Kubuntu 11.10

[Pablo Vegezzi]

Hello,

I am new with Kubuntu 11.10. It's a desktop (Intel Core2CPU 1.86GHz 
RAM-6GB) without problems since some months. 

Some days ago Muon (similar to Synaptic/GNOME) installed new packages 
and asked me a re-boot of the system, but I can't restart since then.

Now, the restart is the normal classic graphic screen showing user's 
name and asking the password. After the password and pressing Enter 
it appears instantly and briefly a 'white window' on top left of the 
screen, and despairing the windows reapers again the classic graphic 
screen asking the password.

If user name or password are incorrect the systems recognize them and 
colors the space where to enter the data. There is no 'white window'.

Choosing the 'Console Login' (Switching to console mode will …) I 
have access to the line commands as sudo or root.

How can I return to my graphic Kubuntu?

Thanks.
Pablo Vegezzi