At first when i boot my PC the login window for my ubuntu is fine... Then i login my username and then properly proceeds to the ubuntu desktop environment.

The problem is when i try to "logout" and/or "switch user". The screen fades out and displays a gray screen. Then the login window is not displayed anymore.

I tried to wait for a long time but still there are no signs of the login window anymore. That's why users can't fastswitch anymore

Does anybody know how to fix this one?? thanks a lot..