I hit this problem after installing a second keyboard layout. I was able to reenable the right Alt key by going to the Keyboard Preferences -> Layouts -> Layout Options -> Key(s) to Change Layout and unchecking "Both Alt keys together". Go figure. From what I can tell, using both alt keys to switch the layout doesn't work, even if that box is checked.
I hit this problem after installing a second keyboard layout. I was able to reenable the right Alt key by going to the Keyboard Preferences -> Layouts -> Layout Options -> Key(s) to Change Layout and unchecking "Both Alt keys together". Go figure. From what I can tell, using both alt keys to switch the layout doesn't work, even if that box is checked.