The same thing happened to my Calamondin Orange tree
about 3 or 4 yrs ago. When brought inside for the winter,the leaves started to look awful, dull, some dried and yellow. MANY small fruit left never fully developed and they all fell off
Now I keep it throughout the winter off the kitchen in a heated garage in a north window. It's doesn't get over 50 degees and sometimes goes down to 40! But it does well, no leaf loss and it looks nice and green and healthy, I guess there is more humidity in the garage and that helps!
I bought a very bushy Meyer lemon tree
and tried to over-winter it inside. NOPE! it also started to lose all it's leaves as well. The poor thing is down to 3 leaves!!! But it has what looks like a few tiny little buds coming now, so by next year this time, it should grow lots of new leaves leaves like the orange tree
If you don't have a heated garage, try a spare room where you can cover the heat vent up some. As long as it's cool and not freezing and has at least a front row seat in a north window, it should do ok and eventually recover by next fall.You might have to some pruning in the spring
Be careful on the watering. In a cool room the soil stays moist longer, so only water when dry. I think I water every 2 to 3 weeks.