I was just about to send a PM and discovered that I had two unanswered queries from other members waiting for me - further investigation showed that notifications for new PMs was turned off - so I have turned it back on - I have no knowledge of turning this off myself, though I could easily have, I suppose - I just wonder if it maybe reset at some point and it might be worth others checking theirs.
Settings - my settings - my account - general settings - private messaging - email notification of new private messages ...I think.