And welcome to LinuxQuestions.org. There are many ways do turn the computer off (pressing the button ain't a good idea... lol). You could:
1 - Fire up a terminal, and type "su -" (to become root) then "shutdown -h now". the "-h" stands for Halt. If you set -r, you will Reboot the machine. If fails, try only "halt" also without the quotes.
2 - Which desktop manager are your running? Because I remember Gnome had and option to shutdown the machine.... That would be easier