Q.House plant
I have had a house plant for almost 15 years. Incredibly sturdy, low maintenance, requires limited watering and light. It has maintained beautiful, waxy green leaves and long growing vines for over a decade. Then, in one day, the entire plant, all at once, went limp, then watery and black. The only thing that changed is we moved houses. I am so sad! What can I do to bring it back?

A move is no small impact on an established plant; different light and air, but usually its because the watering gets neglected for period of time.
It doesn't look good, but the only thing you can do is continue to water as you were before the move, and wait a month for a growth response from the roots.
Superthrive is a good rescue remedy:
https://www.domyown.com/superthrive-the-original-vitamin-solution-enhanced-with-kelp-p-9474.html