ac renault megane 06 - modification last year - now working again?

I had the ac modification carried out at the dealers June 2009 after taking the car in at least twice before when the ac stopped working. The ac has now stopped working again ie light is on but no cool air. After assessment during MOT and service informed that the ac has gas but is just not kicking in. Probably have to approach the dealer again but looking for information before I do this. Can anyone help? Similar problem? Thanks Sheilah