corsa b 1.4 cdx air conditioning?

air conditioning has stopped working on my 1.4 corsa b cdx. All the electrics appear to be o.k. but the compressor does not pull in and the fan does not start when you switch the air conditioning on . I have been told that it needs regassing but would that happen just like that . All this has occured since a garage changed the drive belt and they are the ones saying it needs regassing..