It's happened to me a few times. First time was on a long haul flight, after a meal and while queuing for the loo (some good attention but no upgrade!). Second time was during the night when I got up for a drink - I came round on the kitchen floor. Third after a large lunch I was standing at the bar waiting to pay. On that occasion the paramedics were called but gave the all clear. Fourth time just after breaking my wrist and waiting for my neighbour to come and take me to A&E. She came in to find me out cold on the floor and I came round to hear her talking to the 999 operator saying that yes I was breathing and then to find 2 ambulance paramedics giving me the once over.
On all occasions I knew nothing until I came round and then recovery was quick. Put down to blood pressure lowering after eating, during the night and after shock.
Petition on Why the old state pension is £200 pm less than the new