Had a project with no major activity over the holidays (~2 months). It’s hooked up to Github, and was having no issues.
Now that it has activity again, builds simply don’t trigger on commits to Master anymore. Confirmed that there are no authentication errors or anything like that.
Looks to me like auto-build is disabled for the one build target in your project. Typically, after 30 days without build activity we automatically disable auto-build and send an email informing you of that fact. You can just go back in to the target settings (under Config, then Basic Settings) and turn back on auto build.