Saving Error

I spent 2 hours gathering clay for my base and quit for the day, I started playing the next day and all that clay that I harvested was gone It wasn't in its chest! I play single Player so no one stole it. I Looked in all my chests and it was in any of them. so I did a little test I cut down some trees and placed the wood in a chest, then I exited out and loaded the world Again. the Wood was gone. I exited again and loaded it back up and the Wood was still gone but the saplings that I planted to get the trees reappeared. Please fix this bug its really annoying.