I would keep them separate for a couple of days and s-l-o-w-l-y
reintroduce them. Let each cat have a separate room. Rub a towel on
each cat and leave it for the other cat to smell. Feed them treats
under a door so they can smell each other. Start swapping the rooms
they are in so they can get used to the smells.

Do you know anything that might have gotten one or both of them upset?
Strange cat or other animal outside? A visitor or new person?

Take it slow and be patient. I have been through this and it takes
time. You can do it!