There are a number of things that can cause fur loss in cats.
It could be a fungal infection like ringworm. If you have a black light, use it to check her over in the dark. If her bare spots glow apple green I'd proceed as if it were ringworm, although that's not a 100% accurate test.
Do you see her scratching or biting at herself? It could be mange, a parasitic infection caused by the demodex mite. Or it could be an allergic reaction to something in her environment. Stress can cause it, too.
A trip to the vet really is called for, if there's any way you can manage that.
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