problem i faced due to hanging...
1st my laptop got hanged ... i pressed the hard reboot button...
it could not do file system check for repair my disk (fsck failure) so i was directed to command prompt with whole file system accessible in read only mode.
I thought there is some problem with x servers or ubuntu-desktop package
so i tries to install ubuntu-desktop
1. initx
2. startx
3. sudo apt-get install ubuntu-desktop

then i read error message carrefully so i did shutdown -r now. which restarted my lappie in correct required GUI mode.

Thanks