Ubuntu Logs off on shutdown, Proper shutdown/restart too doesnt work

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http://askubuntu.com – I don't know what i did or what happened? but my ubuntu logs off on clicking shutdown... running the shutdown or reboot command too doesn't shutdown.... how can i find the log? all i know is there is a message saying .... Power_table_CMD wait for 500 sec and something similar it gets stuck on Ubuntu Plymouth... I have to long press the power button on my Laptop to shut it off (HowTos)