Great find! It takes years and years for a jade plant (Crassula argentea) to get to this kind of size, so it's worth trying to renovate it.
Luckily, they don't mind being cut back, even into fairly mature wood (which doesn't get woody, only more aged) so feel free to cut it back to a good solid trunk. Decide what you would rather have; a grove type growth of many trunks, or one or two primary ones. If there is one larger piece, you can use a stick to train it to be the leader. Cut everything else off. More detailed information on pruning succulents here.
The good news is that you can use those pieces to root more new plants! If you don't see any way of salvaging the original plant, don't feel bad if you have to take huge cuttings off the best parts, and discard the rest.
Luckily, this plant, even though you've said that it was neglected and hidden away in a dark place, looks remarkably healthy. This will be a fun project, and one that will provide you with several really nice plants as well.
In time, you will need to repot it, if you want to keep the original part, but that can wait - these plants can survive in a small pot with limited soil for a long time.
May 22, 2021
Thank you by: Norma
Thank you for the excellent feedback! Looks like I am going to own a few extra Jades by the end of this! One other question - even if I look at it, the leaves drop. Is this normal - seems very fragile.
May 22, 2021
Yellow Leaves? by: Jacki
Norma, do the leaves turn yellow or tan before falling? Do they seem water filled or mushy? This could mean that you've over watered it in the wrong stage of growth. Back off on the water, and always temper the water (let it come to room temperature) before using.
Other than that, this happens to many plants when they're moved from one environment to an environment with different conditions.
If the leaves are still green and normal looking, you can root those too!