A similar thing happened to me when one of my cats saw a strange cat on
the patio and then attacked her housemate of 4 yrs. She kept growling,
hissing and attacking the other cat.  It took a long time with plenty of
tuna for them to finally get to living together.  Seprarate them into
different rooms with the door left open a tiny bit or better yet a screen
door or baby gates in place of the door.  That way they can see and smell
each other but not hurt each other.  then place tuna or a favorite treat
next to the door/gate so they associate something good with being near the
other cat.  I also used a portable kennel, putting one in side and leaving
the other outside both got tuna treats.  then switched places and more
tuna treats.  Also when the one who is hissing/growling etc doesn't then
praise him/her.  It took me 3 months of doing the kennel and tuna thing
before they could be together in the same room without one in the kennel.
It takes patience and time but they should eventually at least tolerate
each other again.

Hope this makes sense and good luck!