There's usually a reason for cats suddenly not getting along. Has anything changed recently? Moving? A new addition to the home? Could one of them be feeling unwell and getting grumpy? Hot weather? It really helps to try to pinpoint what might be causing them to fight, if you can.
Basic things like making sure they have enough individual space for eating, sleeping and one litter box per cat can help. If there's a chance one of them isn't feeling well, consider asking your vet. Cats are great at hiding symptoms, and sometimes they're so subtle that they're easy to miss.
They also could just be fighting out of boredom, so try adding some interesting new cat toys, scratching posts or spend some time playing with them each day to try to distract them from fighting.