Turn out the lights or dim them greatly. Sometimes they faint from stress and the shock of bright lights: if you have actinics, they would be ok. Just be quiet around the tank. You may or may not have a fish by morning, but there is little you can do but keep him quiet at this point and try to let him come out of it.
Liveaquaria has an arrive-alive/stay-alive guarantee. I don't want to be a pessimist, but I fear you may have to invoke it; and at least put them wise to what happened, even if he does survive---and he might well. Tangs are known for panic.