a few things i’ve done have caused this, but not sure which was the seminal moment;
deleted all empty tracks -> no playback on timeline.
fearing imminent crash, saved, then saved again with new filename and exited. mistake.
on that occasion was forced to open a recent autosave, but still lost time redoing work
all in the same few mins;
grouped and disabled several audio clips on one track
imported an mp3 that had been extracted from avi to mono .wav, then converted into stereo mp3
placed the mp3 onto an empty track
-> no playback
retraced steps using ctrl+z til playback worked again.
at the time i was doing the above, i was working my way down through multiple tracks, isolating each as i went by way of the mixingn panel. wondering whether the problem might have had somethign to do with isolating one or more tracks in the mixer and then un-isolating them there. if so i have not been able to reproduce the effect.