What should i do if the band came off one of the brackets of my braces?

Plastic color ring: To an Orthodontist a "band" is a metal ring that goes completely around the tooth. It sounds, however, like you are describing an Alastic, a small plastic colored ring/donut that holds the wire into the bracket that's attached to your tooth. In either case, call your Orthodontist. It's a new problem for you, but not for them. They will advise you if you need to be seen now or to wait for your appt