Internal structure failure

This mattress was not cheap. We absolutely loved it for the first year. Then one morning I woke up with a sore back. That night when I went to bed I felt like I was being "sucked down" and I couldn't roll over. Woke up the next morning with a sore back again. That night when I went to bed I noticed that when I got in bed I definitely "sunk" down into the mattress, I tried to reposition and felt a "wall" between my husband and myself and I noticed he seemed "higher" than me. I asked him to roll towards me and when he did he FELL ON TOP of me! The mattress was not one complete unit but more like a pair of twin beds under a memory foam pillow top, We both got out of bed and my side never raised back up! The side I slept on had COLLAPSED approximately 6 inches in on itself! The outer rim of the mattress did not cave in. We are now taking turns sleeping between the bed and the sofa since this mattress will not accommodate 2 people. We are not rowdy kids in bed and we are not overweight. The mattress was deilvered new from the store and hadn't been moved or handled since it was purchase 18 months ago. I am still attempting to contact the company. The store I bought it from has closed.