If you're in the earlier stages of treatment and a bracket comes off, it isn't an emergency as long as it isn't causing you discomfort, scratching or poking, and it can wait until your next check-up. If you've had your braces for a while when the bracket comes off, it's probably best to have it rebonded sooner. Of course, your orthodontist will give you instructions when you get in touch.
This answer is for general information. Your own case needs an examination by an orthodontist.