because the bulb has blown!
this happened on mine last week - you can easily remove the centre console facia and replace the bulb:
1) remove the silver strip of trim from the dashboard between the glove box and steering wheel
2) remove the whole centre console facia by giving it a tug upwards just above the cigarette lighter and cup holder then gently pulling it away from the dashboard (where you removed the trim)
3) you should now have access to the back of heating controls (if not you can pop the heating controls out of the centre console facia but I wouldn't be too confident about telling you how to without breaking the clips
4) there are 2 twist in bulb holders each holding a tiny bulb, the top one does the lcd display - the bottom does the button lights - turn the one you need to remove anti clockwise and it should come out
5) I couldn't find the spec of the bulb (it's so small) but found an identical loocking one in Halford complete in bulb holder (1.2V £1.99), gave it a go and works fine