So what happened is that I was going to use a used up wooden pickaxe as fuel, then the game crashed. I thought it was my laptop because it was lagging a bit. I logged back into the game again and I noticed my furnace was gone. I didn't complain much, so I went to maker another one. I repeated the same thing again by using the wooden pickaxe as fuel, but then it crashed again. However, I wanted to see if it was just the furnace, so I put coal in the furnace and it crashed as well.