i have dell laptop.i am unable to login with my login name.then i pressed ctrl+alt+f1.
then i entered my login name and password .this is not in gui window.
then to get back my gui desktop i entered
sudo apt-get clean
i got error message as 'unable to resolve host yaswanth'
and i am unable to shutdown.
please help me to solve this problem...

Very unclear. Give info about distro. it is GNU/Linux Debian?? or what?. Please explain clearly. When you tried to logging in not using GUI, were you able to login or not?. Why you are trying to apt-get clean?. When you say "unable to shut down" tell how you are trying to shutdown e.g command?? pressing the power button??? Make it more clear.

Also show the contents of /etc/apt/sources.list if it is debian

Are you saying that you are unable to login through the graphical login, so you logged in through the first tty,... and there is something running that is preventing shutdown?

If that's the case,... login to the tty, as you did,... type top at the command line, see what's hogging system resources,... quit top with the Q key,... issue a killall -9 followed by the name of the process and again hit enter,... That ought to kill whatever was running and hogging resources. You may have to issue the command with sudo if the application or process is owned by root.

To shut down without using the graphical interface,... you can issue a sudo shutdown -h now command,... That should kill any running processes and turn the machine off...

try
or F8 to go back to Gui
or you can just shutdown when logged in console
Or root