I just survived the 3rd horde (21st night) but the zombies smashed a few of my containers (desk/box) that held a lot of the stuff that I had collected. I don't see it on the floor. Is it gone forever?
Upon destroying a container with items in it, it will leave a backpack on the ground. This backpack can be moved by running into it, and it does despawn after about a minute. I've tested this in my game using a sink that contained some loot. There is also a small Steam post of someone who experienced the same thing as you. It appears that you've lost all of your items unfortunately.