Cats don't shed completely during seasonal changes; they usually don't shed patches of hairless fur. At most, shedding will make their fur thinner, but this is rare, and we rarely notice it.
If your fur is missing, consider whether you were injured before but didn't notice. Now that you're molting, your fur isn't as thick as before, and the old wound is exposed, causing the hair loss. Alternatively, you might have recently been in a fight or scratched, removing the fur in that area. In this case, don't worry about it; the fur will grow back on its own. 
Another possibility is that the cat has a skin disease. There are several types of skin diseases, such as ringworm, parasites, ear mites, and black chin. In this case, it's necessary to observe the specific symptoms to determine the exact type of skin disease and then treat it accordingly. The cat's fur will grow back after recovery.
