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Try breaking the branches; if they are dead, they will snap. If they bend without breaking, they may still be alive.
Ficus are very fussy about being moved and even more unhappy with repotting.
Unfortunately you may have lost the plant, but wait it out a bit more and do not move the plant.

https://www.gardeningknowhow.com/houseplants/ficus/ficus-losing-leaves.htm

Water is life for a plant, so if it went dry while you were away that could be the primary reason for decline. I suspect more than one week of drought stress.

Sunburn is unlikely unless you moved it from inside a covered conservatory into the sunlight at the same time as it experienced water deficit.

Put the saucer back under the pot and give it a good soaking. Wait and see if it puts out new leaves from the stems and re-foliates.

By the way, from the photo I see girdling (circling) roots at the base. These can constrict and cut off uptake of water and nutrients. This could be part of the problem.

The gnats are called Fungus Gnats. They are not harmful, and many consider them beneficial.

These insects only come out of the soil when it is too wet, which suffocates them. Their food source is fungus, and rotting roots. They do not harm living plant tissue.

With that being said- I think that your plant is being overwatered. Ficus are notoriously drought hardy (Especially the ornamental indoor specimens) and very susceptible to infection.

As long as you let the soil dry out very thoroughly (let's say bone dry down to at least 5 or 6 inches) it will be happy, even in lower light conditions. It does enjoy bright but indirect light, though it can't tolerate full sun for very long.

This article will help you to care for these trees: https://www.gardeningknowhow.com/houseplants/ficus/ficus-tree-care.htm

This article should help:
https://www.gardeningknowhow.com/houseplants/ficus/ficus-losing-leaves.htm

It's always best to examine your plants for pests before bringing them inside for the winter.

https://www.gardeningknowhow.com/houseplants/hpgen/acclimate-plants-indoors-winter.htm

For brown spot disease you can treat with neem oil. Leaf drop is a ficus tree’s reaction to stress, such as under or over watering, low humidity, not enough light, moving the plant or repotting, drafts, temperature too hot or cold. Below is more information on correcting those issues.

https://www.gardeningknowhow.com/houseplants/ficus/ficus-losing-leaves.htm