Sometimes you have to face the negative consequences of working in a team. Well this is not the case for everyone. But, I faced the same problem, the module definitions was deleted by some of the team member and I was scratching my head to figure out where it disappeared?
Well, after track back I figured out that the tabs appearance settings are stored in the DNN's Tabs table in database.
So, I investigated that the IsDeleted value was set to true for the Module Definitions tab. Setting the value back to false worked for me :)
