Whether the cat’s hair will grow back or not depends on the cause of the hair loss(was it a hair clipper, or disease?).
However,most of the time, cat hair does grow back! It may take up to 3 months for short-haired cat breeds and up to 6 months for long-haired cat breeds, but the hair will usually return.
The key to hair regrowth is addressing the infection or condition that caused the problem as soon as possible and using the proper medication or treatment. Unless it is the result of a haircut, in which case, you just have to wait!