All new software has glitches and Pinterest is no exception. You’ve been working hard adding pins to your boards and overnight your boards have disappeared. The pins are still there (click on pins and you can see them) but the boards have gone.
When you try to add a new pin to an existing board, only the ‘visible’ boards appear. If you create a new board using the name of an existing board, it tells you ‘there is already a board with that name’ but it won’t let you pin to it!
Pinterest is aware of this issue and the latest update is that their “engineers are working on resolving it as quickly as possible.”