Welcome to the forum.
Are you saying the ground water at one time was higher than the pool water thus opening the valve? Then the valve stuck open and the pool drained? I may be totally confused and off base but I thought these valves were to prevent the pool from floating if the ground water level rose too high. If this is true then I question how the ground water level changed so fast. I'm not at all familiar with these things so I won't be any help but I am curious about the scenario. Sorry.
Al