Hair loss on just the tail is generally nothing to worry about.  It
can be caused by stress, such as a change of environment or the
arrival of a new animal in the household.  Even the normal seasonal
coat change [8.4] can be enough stress to make your ferret's tail go
completely bald, and sometimes it will take several months for the fur
to grow back.  Often this seasonal "rat tail" shows up with tiny black
spots [10.7].

If your ferret is losing hair other places, there's something wrong.
Apart from shedding, by far the most common cause of hair loss in
ferrets of any age is adrenal-associated endocrinopathy, a serious,
but treatable, disease of the adrenal glands.  Even if the hair comes
back at the next coat change, it's probably still an adrenal problem.
There's a separate Ferret Medical FAQ for adrenal disease, which you
should take a look at if you even think your ferret might have this
problem [1.1].

Other possibilities include poor nutrition, fleas, a severe mite
infestation, a bacterial or fungal infection, dry skin, or allergies
to food, detergents, or cleaning products.