So, today I had half my notebook erased from how it weas yesterday. Luckily I could retrieve it with the ‘Revert to’, but it was a big scare.
It was the middle part of the notebook, and the vanished sections still appeared in the index. Moreover, when trying to select the whole notebook with ‘shift+down arrow’ it stopped when it got to the cell where there was missing stufff. Maybe it was just a visual bug preventing cells from showing?