Reaper sound just quit

[nightofjoycafe]

Been editing all day with no issues, got to a point near the end of the project, and somehow the sound across all tracks has dropped out, still seeing all the correct activity across the track bars, but it's completely mute.

All other sound generating apps on my mac are coming through the speakers as normal.

Settings in Preferences > Audio > Device > default system devices

I'm wondering if I've accidentally hit a key combo and switched something on or off.... really need to get this job finished, can anyone suggest anything for me to try?

Ive seen some posts about plugins. I only have whichever plugins I think came with Reaper when I installed it (V 6.75)

Thanks.