Pool losing a lot of water fast

[Reembeam]

Hi there our inground pool is losing alot of water over night. The pump is off with the skimmers vac point and sump valves closed. We was filling the pool up for a few days the water level got just below the skimmers then that night we noticed the water level dropping very fast and over night the water has dropped below the lights and any other fittings and continued to drop. It has stopped now and Theres barely any water in the shallow end and waist height in the deep end. In the sump we have got 2 screw in plugs one on the floor of the sump facing vertical and another in the sump wall that is horizontal the one that is horizontal is un screwed and the vertical one is screwed in we don't know what they do. if any one can suggest anything it will be much appreciated thank you.