This machine is very old but has given me no trouble until now. I went to carry out my twice-yearly defrost and turned off the freezer as usual (leaving the frig part on - again as usual). However, the freezer did not defrost - it was off for more than 5 hours with no loosening of the ice - it usually takes no more than 2 hours to completely defrost. I noticed that the outside of the freezer cabinet was very hot. In order to defrost the freezer I have had to turn off the whole unit ie frig as well as freezer and it now seems to be defrosting. Any ideas what's wrong?