Why is my Fatsia Japonica dying?

The yellow then black routine is what always happens when some of the leaves die off on a fatsia - it does shed leaves in this way quite naturally.
What is odd is the wilting - have you examined the plant to see if there's any infestation of any kind anywhere, or anything in or on the soil around it. These plants are usually trouble free and can be cut back when necessary, though you say you cut it back "a few months ago" - was it winter, how much did you cut off and what symptoms did you notice when you first saw something wrong?

I Cut mine back all the time, but a few weeks ago mine was looking a bit sick and when i had a good look it was infested with blackfly and greenfly, probably the humid weather, i sprayed it and its looking healthy again..

i didn't cut too much off, it was april i think. the first thing was that the leaves were dying, so i took them off and cleared them away from the bottom of the plant. but it's just got worse. the main stems are long but there's just growth at the top (it used to be bushy) and that growth is wilting and changing colour. thanks for the quick replies, by the way.

If it gets to the point where you decide to remove it (and it sounds like it will) examine the roots and the soil around them after you take it out to see if there's anything there that caused the problem.