I have a "Twisty" Ficus tree that has survived my wife for 17 years. This is the only plant/tree she as ever kept alive for more than a month. It is in the wrong place(6' from the main door), has seen numerous drafts, temperature changes, lack of watering, kept in the same pot, and no fertilizer for over 17 years. Despite all this, it has survived. Vegas had it 20-1 it wouldn't survive the first year, let alone 17. Every once in awhile it would lose some of its leaves, and I read that was normal, especially considering where it's located. Suddenly in the last few weeks since Christmas, it started to dry up and lose all its leaves. The leaves are not healthy, but all dried up. Watering has remained consistently bad, but the same its been for 17 years. Has this Ficus finally succumbed to my Wife's anti-plant terror of treatment, or is it saveable? Any suggestions welcomed and we have a medical team on standby...
Dan, you already have some good suggestions in the comments but here goes anyway. Sudden leaf collapse can be caused by a violent change in environment such as an overnight outside in frosty weather, or more likely as @Giacomo says, a root issue. It's highly likely that some chemical has been added to the root ball, a salt of some kind that has nuked the delicate root hairs so the plant can no longer take up enough water.
With a healthy plant if you remove it from its pot you should see white root hairs on the outside of the root ball. If everything is brown and dark then it is calling for an immediate re-pot. Your goal should be to loosen up the root ball to get the old soil out as much as possible, expose inner roots which may still be viable and put them in contact with fresh potting compost. At the same time you could prune back the amount of foliage to reduce the demand on what is possibly a damaged root system. Hold off on fertilizing (which is itself a salt) until it recovers.
With my rosemary plants in pots I regularly each spring take them out of the pots and hack off the bottom one third of the root ball, apply more compost in the pot and away they go again.
This contribution recommends drastic surgery. You would be well advised to wait for a second and third opinion.