It could be simply that something has got inside the slot and is preventing the card from seating properly. Shine a torch in the slot and see if you can see anything obstructing it.
If not then its likely either dirt on the contacts or the cable inside has come loose, in which case it would have to be pulled apart.
One clue might be how it failed, did it stop suddenly following a knock or drop or did it slowly become less and less reliable.