My Ariston upright freezer will not come off the -24 setting after defrost?

Have tried switching it off by unplugging it,but goes on to -24 as soon as I switch it back on and stays at this temperature.It has been like this now for the past 4-5 days.Even if I try to put it on to -18 it still goes to -24 .I have been told that the gas for this freezer is no longer available and that if it goes wrong it is not repairable.Any help anyone please .