Fence stone

Yesterday, around 1 pm, I was at home in the bedroom and I heard a loud bang. I didn’t immediately realize what had happened, but when I went down into the garden, I realized that the back fence had collapsed. I don’t know how it happened; I only heard the bang. I’m not sure if it was because of the rain, but there is a risk that it could collapse even more. If my husband or I had been in the garden at that time, we could have been injured by the very large stones that fell. My question is: who is responsible for carrying out the repairs? Is there any gas pipe running through that area? When it collapsed, it sounded like when you hit a window hard with your palm.