I don't claim to be any expert but theres a whole load of replies about the air con systems which i lloked through when mine wasn't working.
Although a different model of car this thread was quite useful to me and gave a series of things to explore. http://www.renaultforums.co.uk/showthread.php?t=84666
I was advised to get the system serviced as the computers on the grand scenic tell the air con not to start up if gas levels are low. In the process Renault diagnosed a faulty gas sensor which i now have a replacement for and intend to swap out in the next few days.
Hope you get it sorted soon!