Hello, I'm Dr. Deb and will do my best to help you today.
I'm sorry that harmony between your two cats has been disrupted although this is actually a more common problem than you might suspect. In these types of situations, I advise owners to completely separate the cats into different rooms with their own litter boxes, food and water if at all possible. The process is similar to when you'd introduce a new cat into the household. There are a number of very good articles on the internet about introduction of a new cat when a resident cat is present but the process generally involves patience and taking it slow....don't rush the process, in other words.
Separating them for a while (several days at a minimum) is usually enough to get them to calm down and resume their normal behaviors towards one another although in some cases, it can take longer, even several weeks.
Sharing scents from each other can often help while they are separated, but only if they aren't getting agitated if they first sniff the object. Rubbing a blanket from one on to the other, for example, can also be useful.
Then select a somewhat neutral area in the house to cautiously reintroduce them to each other.
You can also try to distract them with feathers or laser lights, during this phase... try to get their minds off of feeling antagonistic towards one another if there's any sign of hostility but the reintroduction should be as positive as possible.
You might also try a Feliway diffuser or spray; these are natural pheromones which can help instill a sense of calm and reduce anxiety.
There is also an oral product called Composure Chews which is formulated to do the same thing...to calm a cat.
I've had some owners who have had great success with these types of products and others who feel they are not terribly effective....so you won't know until you try them whether or not they will work. But I would definitely consider them; they are available on the internet and some pet stores might carry them.
I hope this helps and that they will resume their normal loving behavior towards one another. Deb