I've seen this happen a few times, the SD card slot on the Raspberry Pi is on the bottom, and if you press down on that part accidentally, some of the plastic can snap away.
That leaves the slot so that the card isn't held in place any more, and can just drop out.
I've seen a few solutions to this on the net, some involve glueing the card in place or sticking bits of card or plastic over the socket, or soldering a Micro SD adapter in place. The better solution would be to replace the socket, but I hadn't been able to locate a suitable socket from RS or Farnell etc. I did find some on ebay, so bought a couple.
These look to be the right thing, so I removed the original socket and cleaned up the board.
The new connector is removed from the packet.
Then soldered in place.
The card is now held in place like it should be and this Pi is now back in action.
Ironic I suppose as the SD Card slot is one of the things that I don't have to remove to fit it into a Spectrum USB keyboard, here is the rest of the board.