if it has gradually got worse it could be a gas issue.best thing to do is switch it off for 24hrs and manually defrost it.if you then switch it on and it works it could be a faulty defrost issue BUT if it isn't able to bring the temperature down then it is an expensive gas or compressor issue.be warned if it is the later fault you will have to replace the whole appliance so don't switch it off unless you are willing to deal with the consequences
ck, July 2011