I had 2 brackets fall off at the start of treatment. One of them was on my back molar and I swallowed it
The best thing to do is call your orthodontist and they will usually schedule you in as soon as they can. If they can't get you in for a while, ask if they can schedule you in if anyone cancels in the meantime.
In the meantime, it can be pretty uncomfortable with a loose bracket. The best thing you can do is try to keep it in place with a bit of wax.
Don't worry, even though it has come loose from the tooth It won't fall off the wire