i have skype installed but if i close the "main menu" it is still running in the background but i cant open it again. How can i open the window again?
Skype does currently not play well on ubuntu. A work around in your current situation would be to kill all running skype instances e.g.
pkill skype or
killall skype and then launch skype again.
If you want to avoid this in the future, don't close skype, just minimize it.
As an alternative, you might want to try the skype wrapper as described here. As far as I know, this wrapper should be able to handle your situation as well.
Have a look at this article:
Skype is not white-listed in Ubuntu 11.10 but you can change this..