EWW!

First thing to do is remove all the dead leaves and stems as they become agents for disease if not already infected. Then if there is any life in it, remove it from the oversized pot and pot down to a much smaller pot. Use a good potting soil for the repot. If the soil is dry, then lightly (LIGHTLY) water it - do not saturate. Put it under some kind of humidity dome (a baggie will work but not the best) and put in a well lit area for several hours a day (12 hours is a good number of hours but not necessary either). Check on it every few days for moisture and fungal diseases. If all goes well you should start seeing new growth in a couple of weeks or longer. Remove the humidity dome/cover for a few hours to see how it responds to normal household conditions. If wilting then it is not ready and needs more time being pampered. Eventually you should have a beautiful plant that can thrive in your home. This looks like it was one of the small leaf rexes at one of the big box stores (Home Depot, Lowe's, Walmart) and they can thrive if they get enough light, just enough water to meet their needs, and a little care.

I think your plant may be 'Hugh McLaughlin' who was the president of the Scottish Begonia Society (I think he passed a couple of years ago) and was created by nurseryman Antoon Hoefnagels in the Netherlands. Here is one of my Hugh begonias.

Each leaf can be used for creating a new plant. Here is a small aluminum pan I used to start 20 or so of them in 2016

It certainly stands out in a group of dark leaf begonias in 2014

I combined these mini rexes I bought last year from the big box stores into a 10 inch pot. This picture was from the 17th of this month. I have them under T8 cheap fluorescent shop light running 12 hours a day.

This is still in its original container from the big box store.

I bought this one from Walmart and moved it to an 8 inch pot. The secret is to lightly water so it is not water logged. It is much easier to add a little water if needed than try to remove water from a waterlogged soil mass.

some I started from leaf wedges last spring in Jiffy pellets and moved up to 3 or 4 inch pots this past fall. From humidity trays to living in the conditions of a heated basement with cheap shop lights.

I started 21 plants from one very large leaf last spring. I potted up 15 of those in 3 to 4 inch pots and gave those away at our begonia meeting last fall. The remaining six I kept in the cutting tray for several more months so they basically quit growing (small Jiffy pellets). I finally thought it was time to free up the tray and move them to this 10 inch pot. These will become giants eventually.