what should i do

so basically me and my sister was playing and I wanted to go craft the obsidiron chest. i got the last 2 ingots and because my sack was full i was dragging one and the other was in my sack. i then went to the lava to cook the ore. then i went to the water to cool it off and both were still there. my sister had offered to take some stuff because my sack was full. my camera angle was in the water so i couldnt see what she was picking up and i assumed she picked up the ingot since it was gone. then i went to the anvil to drop off my stuff while she returned to camp. i came back to camp when i realized i was missing one ingot from being able to craft the chest so i asked her if she had it and she said no. we searched the whole map for it and it was just gone.