I've grown Ficus benjamina in Buenos Aires. It belongs to the group of strangler figs that need hardly any substrate at all! They thrive with even poor light conditions indoors as long as they get good ventilation. No direct sunlight. They hate stagnant water in their roots. Like many have said already, perhaps its too late to save your plant. Lost anyway, you may want to place it in a new container with a new fertile substrate with a lot of pebbles and sand in a rather shallow pot. I would prune it and then hope for the best, asap. If unsuccessful you will still have your pot ready for a replacement.
Arturo
