If you suffer from the vanishing OB on occasion as I do, I've found that putting a trapdoor (open or closed) directly under (touching) the OB will help. When the OB vanishes, activate the trapdoor and move within a couple blocks from where the OB is and it comes back. This works every time for me so hopefully it works for you.
If that doesn't work, then get far enough away from the OB chunk so it unloads and then come back and it will be there. Going to /spawn or something and back does the trick.
If that doesn't work, then get far enough away from the OB chunk so it unloads and then come back and it will be there. Going to /spawn or something and back does the trick.
