If you press command (the apple key), option, and escape all at the same time it'll bring up a "force quit" window.
Click on Finder and choose restart. It will then restart finder and you should be able to click on the apple and go to shutdown.
Worst case scenario: if you can't get it to work just press and hold the power button on the pc for 5-10 seconds and it'll shutdown. You can then turn it back on.
