Assuming you hardly ate, you probably lost a lot of glycogen and its associated water. You might have lost a LITTLE fat (1-2kg).
Glycogen stores are around 400g and each gram of glycogen is tied to around 4g water.
The reverse happens when you eat, drink and are merry over the Festive Season. Many dieting posters gain 3±1kg and are most alarmed by their NYE weight, which is usually more sensible by 10 January, if they resume 'healthy eating'.
You don't really notice when you piss away litres/gallons/kg of water...