I opened several project files on 2 separate hard drives and in all cases, I couldn't get them to output audio.
The external insert seems 'broken', i.e. the fx return suddenly has only one option in the dropdown menu: none.
So I deleted the external fx from the 2 bus fx window, still no sound. The outputs are assigned properly. Rebooted, no change.
The only change has been the installation of Slate Trigger in the fx bin.