Trying to restart Mac, but get error msg: ical cancels restart?

I was trying to update softwares which require to restart the computer. An error message appears saying "ical cancels restart", quit iCal and restart from the apple menu. When I click on iCal icon from the task bar, and click Quit - nothing happens. When I go to the File menu, the "Quit iCal" is greyed out. So I don't know how to force quit iCal so I can restart my computer. Any help is appreciated. Thank you.