I have posted this on Fiesta section but no replies - wondered if any Ford owners can help?

I have a 2007 Fiesta zetec and the air conditioning has stopped working. If anyone can advise on what the problem could be it would be appreciated as the car is just 1 month over the 3 year warranty and has a low mileage of under 12k so am hoping that if it will be something that would have been covered under warranty that I can contact Ford to try and get it repaired under a goodwill gesture.
I would like to find out some information before booking it in at a garage to keep the cost at a minimum.
Last year, I think about August, the air con got regassed as when I pressed the air con button you could hear the clicking noise from the engine but the air was not cooling. I was told that it was probably that the air con had not been used enough and the seals had not worked as they should and gas had leaked out. The air con worked after this.
This time when I press the button there is no clicking noise from the engine - the button lights up but there is no change in the temperature.
Is there anything that I can check myself?

Any advice is greatfully received!