A lot of things can cause this. The first one I'd consider would be did the tank get a major chill, possibly due to all the cold weather? This can really mess up the ability of the fish to swim normally.
Another issue can be swim bladder problems. Does the fish tend to sink or float if it's not moving? As a note, some fish that dwell on the bottom don't have a swim bladder, so this isn't an issue, even though they tend to sink if they are not moving.
Has the fish been in a fight with something else, or did it contact anything like an anemone?
Ehat size tank do you have? Tangs need running room snf can damage themselves if they don't get it.