No one can give you the exact reason what went wrong we can just guess it could be a bad private feedback which can effect and result in drastic drop.it could be a bug may be as they are working on it.Before there were problem with My stats page now its ok but I see Nothing happend yet to my JS just the date has changed may be it will go up or down later as I dont trust this robot.it can make you zero from hero.
I think last time you had a problem with your client may be he complaints about you as its also a part of our JS%.
I think Setu can help you with some manual calculations as I am weak in math.
Undoubtedly if you have not had problems with the other factors involved in the JS score calculation (disputes, missed deadlines etc); then negative private feedback is the culprit as Iftikhar indicated.
After the added information on JS score is released, if you still have grouses you cannot figure out, I will assist with as much as I know (or think I know ).