32 amp fuse on battery keeps blowing??

Hi can anyone answer this for me, driving to work in megane 1.5 dci 2004, parked car in carpark, finished work, went to car, won't start, no power to the dash board just nothing, found that the 32 amp fuse on the battery blown, replaced fuse, blown again? and again? any ideas why? don't want to take it to the dealers as i guess they will charge a fortune.