How do you fix minecraft when your blocks disappear?

Answer 1

You will need to be more specific. If the blocks dissappear, but only one at a time, that just means that there are Endermen around, and the only way to fix that is by killing them. I suggest creating a 5x5 platform two blocks above the ground and standing under it. From there, put your cross-hair directly on an Enderman for at least a second. This will cause the Enderman to teleport nearby once you look away. Once it teleports to the outside of your platform... Murder it. Collect Ender Pearl... Be happy.
Otherwise, it could be chunk errors, but if this happens multiple times, it is more unlikely. If you are on a server, it could be grief, meaning that other players break or take placed blocks.