Welcome to Reef2Reef and sorry for your troubles!
How big were the clowns when you got them? Was one of them significantly larger than the other? Did you have any changes to anything in/around the tank such as the lighting?
Clowns are protandrous hermaphrodites (meaning they start as males and when they become the dominant specimen in a pair, they turn female), and when they turn female they can sometimes become highly aggressive.
Additionally, clowns use color and UV reflectance to determine social value/hierarchy (a lot of people phrase this as "clowns are racist"), so with drastic color differences like one primarily white clown and one primarily black clown, there can sometimes be major conflict between the two - I've found these issues can be sometimes be exacerbated by specific lighting conditions (i.e. more blue light or more white light, dependent on the individual clown pair). So, these issues sometimes pop up when someone changes their lighting up.
Sometimes, when clowns get too aggressive, people just keep them by themselves and stop trying to pair them, but sometimes the clown will accept a new partner. If you decide to try getting it a new partner, I'd suggest getting another snowflake (similar color morph to avoid any "racism" from the current clown) that is substantially smaller than your current clown (two females will fight for dominance), and I'd put the new clown in a breeder box in the tank where it can see and "smell"/be seen and "smelled" by the current clown, keep it in the breeder box for at least a few days, possibly as long as two weeks, then release it into the tank and observe for signs of aggression. If the current clown still shows aggression, you may want to remove the new clown and give up on pairing the current one (at least for a while).