I logged into a world with only a few days on it and went to build some refrigeration units. When I got to placing the wire to power them, I noticed that the category was grayed out, the way it is when nothing in that category is buildable. Further investigation showed that I was unable to build anything requiring metal ore, and the inventory did not show any metal ore. I swept a bunch off the floor into storage to see if that would fix it, as well as breaking more copper ore, but none of it started registering as being available to build with. The attached screenshot just shows the category being grayed out as well as some copper laying in the background.
I simply logged in and noticed that it happened. I am not sure what caused it to happen, only that reloading the world did not resolve it.