Espace AC Problems

I have a Renault Espace 2001 RTX with Air conditioning, only problem is that the unit decides when it will come on. Clicking through the buttons I hear all relevant valves and vents move but no air either hot or cold. At my wits end as have 2 kids and we cant go anywhere if it has been raining or a cold day - HELP!!!! Hopefully it is something stupid I have over looked but dont know enough about engines etc.

Stefan Hayward