Newbie dilemma: Spa attached to the pool suddenly lost water

[waste]

Welcome to the Forum!

If you can provide the pics Ben asked for, I'll do my best to tell you how things should work and what might be wrong.

[waterbear]

There should be a check valve to keep the water from flowing out of the spa and back into the pool. The flapper mechanism inside them does wear out and need to be replaced from time to time. That would be the first thing I would check.
Your post also seems to indicate that you have an automation system with valve actuators turning the valves. It is possible that one of the actuators has malfunctioned. or the valve itself is leaking.
As both Ben and Ted said, pictures of your setup would be most helpful.