Sometimes there are hidden coils in the freezer. An electric heater on a
timer will defrost these. If that is not working, it will be a solid block
of ice.

Try putting all your food in ice chests and let it defrost for 24 hours with
doors open (unplug it). Then plug it back in and see if it works.

If it works for a few weeks, then same problem again, then suspect the
defrost heater/timer.

As to fixing, if it is an old refrigerator, get a new one as those save
electricity and will pay for themselves in 5 years or so.