MY FICA BENJAMINA IS LOOSING ITS LEAVES

Something (ficus benjamina) is known for (shedding leaves) when moved to another light source, sometimes for month on and off, or, purchased within the last three months, not watered correctly or has a pest problem.

Make sure you find a good bright indirect spot with a little direct sun during the day and live it there. You can turn it for even growth without any problems, just don't keep moving it to other places.
Water when the top couples inches dry and has good drainage and only feed half strength in the spring and summer months and ONLY if it's putting out lot's of new healthy growth.
Fertilizer will "not cure" any stressed or ailing plant, it can make thing worse. Only fertilize healthy growing plants. No fertilizer needed in winter since they pretty much stop growing.
New leaves should grow back in time. Just be careful not to over-water now that it has less leaves.
You may have to do some pruning of any dead branches after new leaves fill out the tree again.