sudo service mdm stop || sudo /etc/init.d/mdm stop black screen problems

Recently, I encounter a black screen problem when I try to shut down mdm (x server) under command line console.

The problem is finally solved by kill two mdm threads and one xorg thread.

To do so, type
>> top
Then, look for PIDs of two mdm and xorg. Then kill them by typing
>> sudo kill PID1 PID2 PID3


To start mdm, just as always:
>>sudo service mdm start.