It is the refrigerant gas escaping from the pipes or compressor. Without gas the fridge will not get cold. Did you pierce the pipe or the cabinet wall with something sharp or maybe move the fridge and somehow cracked a pipe at the rear? Not usual for a major leak to occur by itself. Motor will eventually overheat and will have to be replaced if you keep running it with no gas. Turn it off & call a repair company to give a quote to regass it.
Lincoln Appliance NZ, September 2009