Why has my freezer defrosted on it's own?

went to get something out of the freezer and realised that it was totally defrosted and all the food was lying in water just. Power light is still on, and there is another button beside it an alarm button you would have to press this button in for whatever reason, I have not pressed this in as yet. Would be gratefull for any answers/ ideas. It is a Vestfrost sz101c c white.