Hard to say without knowing more about it. A common problem is overwatering. Make sure to let the top layer of the soil dry out in between waterings. If there is already root rot, you would need to remove the rotting roots and repot the tree. But as I said, there could be many other different reasons
If you keep ot indoors it is normally fine to repot in any season. However, if you want to play it safe, early spring would be best, before the major growth spurt starts
What has worked for me with ficus microcarpa: - Repot only when the tree is healthy or when repotting is they only way to stop its health from further deteriorating (e.g. root rot) - If kept indoors repotting can usually be done all year around. If your unsure, early spring is the safest time regardless. - Ficus are very forgiving, also when it comes to more severe root pruning (I took 80% off the root mass of one of my trees and it is still going strong 2 years later) - Opt for well draining soil. For me root rot was the biggest stuggle with my ficus trees so far. This has improved enormously since I switched to better draining soil
