Swim bladder problems can rise from poor water quality. If you get a testing kit you can see what the ammonia, nitrite and nitrates and pH are. You only need to worry really about the first two, nitrates are harmless, and platy's like more alkaline water, but will accept a slightly more acid tank.
In the short term, i would do a 25 to 50% water change. That will dilute any harmful toxins there may be in the water. Then look at getting an internal anti bacterial medication. Something like interpet 9 might help.
Unfortunately though, by the time the fish has lost the ability to balance itself, it is possibly on the way out.
