Sounds like it could be a routing problem somewhere.
So the meters are moving on the Instrument tracks as well, but nothing through the Output Track?
And your other projects are working, just not this one?
Find the Logic project folder. Right click on it and choose Get Info. this will give you the information on the size of the project.
You could also create a new project and then import the tracks form this project to the new project. This may resolve the Output audio problem.
Here's an article on how to import tracks form one project to another:
I know I had a problem like this a couple of years back and the only way I could resolve it was by using the import feature and to import the tracks into a new project. I think my project ma have been damaged and this is why there was the Output track problem.
Try i out and let me know.