Thawed fish can be refrozen only if ice crystals are still present throughout according to North Dakota State University. Fish and seafood are highly perishable and can be spoiled even if they do not have an offensive odor.
Thawed meat and poultry that has been sitting for more than 2 hours at a temp of 40 degrees should be discarded. Thawed vegetables should not be refrozen unless they still have ice crystals. Bacteria breeds rapidly on vegetables. and the spoilage can be toxic and hard to detect.
Fruit can be safely refrozen, even if it has started to ferment, but its flavor may be affected. Thawed ice cream and frozen dinners should not be put back in the freezer.