I am sorry for my pool english. : )
My SCIM was disabled under the recovery mode.

I left my notebook home for downloading movies.
After my returning from work, I found that it worked incorrectly.
Restarting, no choice.
Then, the problem came out: put account name and password, and strike ENTER.
The xwindow screen flips to black, and several seconds later it comes back to login screen again...
I had tried 3 times, it was all the same.
I don't know what happen to my system.

HELP!HELP!HELP!T_T