I have a hibiscus tree, about 20 yrs old, potted in a very large clay pot. This winter it is literally dying before my eyes. The leaves on full branches are turning yellow and/or wilting/drying up. It is wintered inside. Should I cut it right back or try to repot it? Is this because of its age? I hate to lose it, as it is quite large. Any advice would be appreciated.
As they grow in pots, plants gradually fill up the available space with roots. When all the space is filled, it becomes difficult to get enough water into the plant, because there is literally no place for it. This is a sure sign that the plant needs repotting. Since you've had the plant for many years, and apparently had no problems with it before, I think may well be what's happening now. Most repotting articles suggest that the best time is spring or early summer, when the plant is starting its most vigorous growth. However, yours sounds like an emergency.
First, be creative in getting more water into it. Can you put it into a large tub or something so that it can sit for a few hours in water, to rehydate? If you can accomplish this, you may be able to hold off on repotting till spring. If not, you're going to have to do it now.
Decide if you want to put it back into the same pot, or if you want to put it into an even larger pot. If you want to up pot, get your new pot ready. Gather up fresh potting soil, and all the tools you'll need.
The biggest job is getting a plant out of a large pot. You may want to have this done by a nursery or other professional, but it can be hard to find someone you trust. If you want to do it yourself, or with a helper, you can lay the plant down on its side, and work it out of the pot. You may have to use a large knife, worked down between the sides of the pot and root mass, to loosen the roots enough to pull our the plant. Or it can stand upright, one person to lift up on it, one to hold onto the pot. Just make sure the roots are loosened from the sides first.
After you get it out, you'll need to prune the roots. If you're up potting, you'll still need to untangle and prune roots, I would think. Here are some articles that should help you: https://www.gardeningknowhow.com/houseplants/hpgen/learn-more-about-repotting-houseplants.htm