The next step is to try to shutdown using Terminal: open it and type
sudo shutdown -r now
and enter your administrator password (you won't see it as you do so) at the prompt.
When it works it will shut everything down cleanly and restart, though I have seen instances where this won't work either.
If you do have to do a switch-off you should do a Disk Repair: the easiest way is to install
AppleJack and run it on the next boot, in single-user mode. My experience is that forced shutdowns always leave some disk directory damage, and if you keep doing this without repairing you may run into more serious problems.
(AppleJack is quite safe: it's simply a basic interface to access built-in Unix processes which you would otherwise access by typing in obscure commands).