Suddenly lost metronome.  no sound.  I have it set for audio.  no sound in recording or playback.  and yes I know how to select 
those options.  this is project specific.  it only does it in one project.  what could I have done???

Hi. If metronome is going to a metronome bus, I would delete that bus (it could have become corrupted somehow), and replace it with a new one.
I have had that happen to me before. Hope it helps.
All the best.

Before deleting the metronome bus, have a look if it's muted, the input gain is at min, the level is set and that the output of that bus isn't going nowhere.  Presumably opening the project would have warned about silent buses.
 
If the metronome is going out direct and you have a multi i/o, might check the mains levels in the Console.  That could be down in the project.
 
Keith

There was an issue with the metronome not sounding in certain projects a while back. Toggling the global FX bypass button in the mix module was the workaround. May or may not help but worth a try.

tenfoot
There was an issue with the metronome not sounding in certain projects a while back. Toggling the global FX bypass button in the mix module was the workaround. May or may not help but worth a try.

Bingo!!  that was it.  thanks a lot!!!

HighAndDry

tenfoot
There was an issue with the metronome not sounding in certain projects a while back. Toggling the global FX bypass button in the mix module was the workaround. May or may not help but worth a try.

Bingo!!  that was it.  thanks a lot!!!

Glad you found it!
 
I believe it was caused by an issue in an earlier release and seems to go away upon resaving after the FX are toggled - assuming of course you are not still using that same earlier release:)