Ah, the worm disappearing act. Worms are wanderers! You'll see this happen occasionally and it's not a worry for the worm farm if they start exploring in small numbers. If they leave or burrow down in larger numbers, this indicates the conditions in the farm are not ideal for the worms. The conditions need to be corrected (e.g less moisture, less nitrogen, more oxygen, adjust temperature, add more food, remove competitors or old food etc.). From there, they can wriggle their way back if they are still in the farm. 

Worms may also cluster at the top of the bin or on the underside of the lid if it is about to rain - think of this as your natural weather reader.