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Start an aggressive water change routine right away. Every three to four days, remove and replace half the tank water with pure, treated tap water. Vacuum the gravel where possible. Raise the temperature in the tank to about 82 degrees and add a little more than a teaspoon of standard aquarium salt to every 5 gallons of the replacement water. Follow this routine for two to three weeks.
If you've caught the "Ich" parasites early enough. These steps should help. Keep me informed if you like.
Sorry about your fish. I've lost a few too. Fin rot is preventable. You likely have some aggressive fish in your tank that stressed your Platy and possibly your tank management, specifically large, frequent water changes need to happen more often.
If you keep "Livebearers", like Platys, Guppies, etc., it's good to dose a little standard aquarium salt in your replacement water. A little more than a teaspoon for every 5 gallons of your new water will keep the fishes' immune system strong and makes for healthy gill function.
Anyway, check your other fish to make sure one isn't bullying the others and make sure the tank water is pure. Flushing a lot of pure, treated tap water through the tank regularly is the best thing for your fish.