Something about its growing conditions are no longer working well for the plant. This weakens it, and makes it susceptible to various infections.
For starters, I can see that it no longer has overhead light, only a little side light from the windows. The leaves are all at the top, and they need light to do well.
This is a problem with keeping this type of plant indoors, they out-grow the space available. I don't know if it will come back if you prune it, maybe we'll hear from someone with more experience.
Can you add a picture of the pot? Have you ever re-potted? How do you water?
I'd say you've done well to keep it going this long, but it doesn't look well now.
