Fatsia losing it's leafage completely

I'm amazed you've managed to keep it for 8 years in the house - this is a hardy outdoor shrub which, after 8 years, would normally be 6 to 8 feet high and wide.
Not sure what's caused the sudden collapse, but naturally enough, it hasn't been able to grow as it wanted to, with enough root room, so perhaps that's a factor - turn the plant out of the pot and check the compost and roots for bugs, and to see if it has any healthy roots left. If it does, replace the compost with fresh, put back in the pot and cut it down and see what happens.

thanks, I will do that, but in terms of cutting ( I haven't done it before) so I just cut it right down at the bottom of the stem?

If there's no sign of new growth any higher up, then take it down to about an inch or two.

ok, one more question, the top leaf is relatively new- just a couple of month, if i cut a few inches from the top as well and stick it in the ground, could this work to start a new plant? I did that with my Dracaena and it gave roots and started growing. Could fatsia behave the same way, their stems look very similar? And thank you again for your advise

Shouldn't work, but don't let that stop you giving it a try, I would!

I would re-pot the Fatsia into a larger container first of all, checking for any rot or pests. Then I would cut back but leaving the remaining leaf for energy.

If you have a garden I would put it outside if you see signs of regrowth. You live in a similar climate to myself, so the Fatsia will do OK in full sun in Ireland. It will grow thicker, shorter leafs. If you put it in full sun the leaves will go dark green, in shade or semi-shade you will get lovely jade coloured leaves. Once acclimatised to the garden they will survive frosts and snow.