I have to assume you don't have a liner as it would have shrunk a lot. Also, the volume of water suggests it isn't a fiberglass shell as the estimated volume of lost water is 20,000 gallons, more that the vast majority of them hold. You have either a concrete or hybrid pool. If you have either, remove the hydrostatic reliefs, be they plugs or valves.
Since the water level is dropped so much, its likely to have gotten out either through a bad drain line, failed drain pot, or a failed hydrostatic relief port that contained a plug or a valve.
If its a bad drain line, a pressure test is called for to verify it. The repair may be as simple as plugging the drain or may involve digging up the floor, which I know you didn't want to hear as that can be expensive.
If the seal between the drain pot and the pool has failed, a little silicone may seal it but it will drop the water when it fails. I suggest it be dried and some hydraulic cement or plaster patch (just about the same stuff but colored) be set to reseal it. Lets assume the water table is below the pool or the leak would stabilize at the water table's height or reverse itself and dirty water would enter the pool.
A bad hydrostatic port may fail either where it meets the concrete and finish which also sucks or it can be fixed by using new hydrostatic relief plugs and valves which is more likely and less costly.
Check for cracks too. Since the water is so low, we can't tell if there are additional leaks, be they lines, cracks, or seal joints where two dissimilar materials meet, like skimmer mouths, or bad plumbing, etc... anything that can leak.
Unless you have the pool under your homeowners insurance, any loses are on you, Water may come from your well if that is what you have, municipal water if that is what you have, or you can have it delivered. Since we don't have your location in your signature, we can't tell you for certain. Add in the costs for balancing. Calcium, alkalinity increaser (baking soda), pH UP (Super Washing Soda), and borates (if used) add up.
Scott
