I started a new project, and I'm about 6 tracks in.  I hit a few hotkeys - stop, rewind to marker, record (inspiration is happening) and suddenly the metronome stops working.  I reflexively hit F3 / Ctrl+F3 and then realize the icons are already lit.  So, I go to preferences and double check the settings, and they are as always - audio metronome, -5 & -5.5 dB, just the way I like it.  I saved, started a new project to check the basics, and the metronome works just fine.  Reload the project and same thing, no metronome.
 
So, did I discover a set of hotkeys that somehow made my metronome inaudible?  Or is it something simpler?  
 
Thanks.
Dave 
 
 

Check the Output assignment of the Metronome in preferences. It should be going to a Metronome bus that in turn outputs to Master (or direct to Main Outs if you don't want Gain/FX on the Master to affect it). Then make sure that bus isn't muted or maybe silenced by soloing some other bus.
 
Since it's working in other projects, there apparently isn't anything wrong with the availability of click samples or anything like that.
 
 

And to add to that, some projects don't have the metronome buss. 
It's easy to make one.
Just add a new stereo buss in the Buss pane. 
Name it metronome
Assign output to interface 1/2 ( or ?) 
Now go to the metronome options and make sure to assign it's output to this buss. 

Thanks guys, I'll give that a look.  It's possible, I guess, I could have deleted the metronome bus by accident in a flurry of hotkeys deleting VST tracks etc.

This happens to me in a pretty much finished project where I decide to re-record or record a new part. Of course when re-recording I turn off all FX. This sometimes causes no sound from the metronome. Turn FX on and off and the metronome should be heard again.