How do I enable Javascript?

When starting Acrobat, I suddenly get a dialog box saying Javascript is inactivated. I have not changed any settings. Since this box is system global/modal, it blocks the underlying Acrobat window, so I can't access its settings.  Is there a simple solution (supposedly one can adjust the setting through the registry, but how do I do that without damaging anything).  And why did this box suddenly appear?  My Acrobat Reader doesn't display it.