Hi Lol, Sorry, I missed you question when posted and as you have had no response I will give you my thoughts on your very sick Fatsia. Looking at the photo, I would suggest that the problem is with the roots and that something is stopping the plant taking up water and that with this recent spell of warm weather, the problem has been magnified. As to the cause, without actually seeing the plant in the flesh it is difficult but I think I would put my money on a very bad infestation of vine weevil larvae, its a little early for vine weevil but if you tug gently at the wilted bit of the plant and it pulls easily out of the soil, you will then find in the compost the white larvae of this terrible pest. I'm pretty sure that if I'm right them the bit that is not wilting very soon will be as the grubs find and eat it's bit of root. If this is the case, shake or wash off all the old compost (to remove all the grubs) cut back hard and re-pot in fresh compost, water and place somewhere cool and you may have a slight chance of saving it.
Regards Lannerman.