When you do a rebuild it stops the existing version and builds a new one. If that rebuild fails, then you will have no app running. You can start the existing, pre-rebuild one again, but that won’t include any of your latest app.yml changes.
Running ./launcher rebuild app usually takes around 15-20 minutes, and your forum will not be accessible until it’s finished. All being well, your rebuild will run with no errors. If it does fail, you should get a message that says:
** FAILED TO BOOTSTRAP ** please scroll up and look for earlier error messages, there may be more than one.
./discourse-doctor may help diagnose the problem.
And if you scroll up there should be more information that can help solve your issue.
Does any of this sound familiar?