If it was a new plant, I could say all kinds of things, but as you've had it so long and obviously have done things right, you'd have to be the judge of whatever is wrong... you'd know what changed better than anyone else... so sorry, but sometimes stuff just happens that we can't understand (not being plants :).

Hmm the leaves are turning brown at the base. I'm wondering if this could be root rot, in which case I'll repot it again, but I'm also wondering if the previous pot wasn't draining, which may also explain brown leaves...not ready to give up on this plant just yet!

Please let us know how things go, if it comes back, what you did...

My first guess would be overwatering. Like moochinka said, you should be the best one to determine if this is true. I know in the past i have been guilty of watering an already root rotted plant because I thought the falling leaves where a sign of under watering. Dig deep with your finger and feel the soil. If this is not the case, cold drafts near a cold winter window sill could also cause damage. Humidity is also a concern in a winter heated house.

FICUS RETUSA can be tempermental to changes in the environment. Try to ease it into changes in the seasons. Bring the tree outside when weather permits. Well draining soil moist but not soggy. Moderate humidity. Bright light. Feed with a balanced fertilizer. I currently use a 3-1-2 ratio npk fert (recommended on GW) for most of my trees now. Also have used a 1-1-1 in the past. I also like a little root fungi (Mycorrhizal) in the mix.

Fyi my little F. Retusa was also dropping leaves this week

Figs that drop leaves a lot of times adapt and flourish again. Observe changes in you tree and act accordingly. Good luck !