I’ve been working on the same project for at least a month.
Project = 1 backing track and 14 tracks of drums only using the metronome as a 60 second countdown. It has been working fine.
Today I renamed the project, set countdown to 90 second, removed all drum tracks leaving only the 2 overhead mics and the backing track. Now the metronome will not play at all on either projects.
 
I had to rebuild the setup project and now the new project works fine.
 
I still wonder what could have caused the metronome failure that cost me half a day in troubleshooting and a project rebuild.  

This sounds like the metronome issue others have experienced.  Try toggling the global "FX" button (Bypass All Audio Effects) in the Mix Module normally along the top of the main window.  The keyboard shortcut is "e".
 
 
Please see this thread http://forum.cakewalk.com/Who-has-the-metronome-bug-m3397876.aspx
 

Thank you for the info :)