You know Daniel, I always find it very amusing when humans, who DO NOT know
the answer to an animal issue, call animals STUPID ! See the paradox? But
despite the name calling, at least you are trying to get some help.

Think about it: what is growling and hissing: a warning generally. Warning
that if you come any nearer, one will not be responsible for his/her
actions. Why do you warn?  Because you are angry, or in a bad mood, or fed
up with something, or tired, or in pain, or scared really. Attack is the
best defence.

There is a number of reasons why a cat can suddenly change the mood. Just
like humans. It could be some sort of stress, she could;ve been scared, she
might be ill, she might (hopefully not)even have a brain tumour. You are not
saying if they are outdoor or indoor cats. There is always a chance one of
them has rubbed/rolled herself into some odd smell, which the other one
doesn;t like or dislike a lot. Cats are extremely sensitive to smells. Or,
she could;ve been scared by a passing anima;/human/track/helicopter. Rubbish
man.My neighbours Barney is terrified of rubbishman. My Charlie doesn;t feel
the traffic noise, but if a Council track that collects large items passes
by, he runs like Hell back in. My Shadow is scared of a doorbell, while
others barely open their eyes to check the situation.  My Tiara will "swear"
at anyone who comes back from the vets, until the smell subsides sometimes
days later! You could've done something to upset her. My Tigger refuses to
sleep with me if I just had a bath and actually smell gorgeous! But hey, who
am I to argue his sense of smell! He can see better, hear better and sense
things better then me anytime! So how can they be stupid!?

The real truth is: you might not find out what caused this, because quite
simply, we humans, no matter how intelligent we think we are, really are, to
use your terms,  very "stupid" when it comes to understanding animals, and
their language. We just don;t know for definite, and we have not found the
way to communicate with them, more then bribing them with food to stay
around.  Most of the time we are just guessing. Throw a cat out on the
street: it will survive! Throw a human out on the street. Will it survive?
Maybe. So who's the smart one?

I have had  my 4 cats for 5 years now, and as much as tried to get to know
them (and boy have a I read a lot) and understand their behaviour, I came to
a conclusion that animals are just as complex as humans. I love them to
bits, and would give anything to be able to understand them more. They all
have their own personalities, own games, own likes and dislikes, own taste
in food, humans and toys. Even weather.  They know how to get what they
want, they know humans much better then we know them, they can be nice or
devious, and they can feel grief if loved one dies. My Shadow spent 2 months
practically not moving from the window when QT died. They were soul mates,
and we he got run over, Shadows life stopped as well for a (long)while. You
don;t own a cat. A cat owns you.

So what do you do in your situation? If it was mine 4, I'd check with the
vet the health of the angry one, and if clear, let them deal with it
themselves. If the situation gets too much, make them smell the same: spread
some tuna brine over them. Let them lick and clean each other, and it may
work. One may have upset the other, so bribe them: give the upset one more
reassurance and treats when other one is around, and only then. It;s called
positive association. Never yell never hit. By locking the upset one away in
a small area, you are reinforcing the negative association. Just like
beating, negative association doesn;t work. Or it does - in a negative way.
And I know you don;t want that.

Good luck

Gee, Tiara, Shadow, Tigger and Charlie
with QT in our hearts