My page grid seems to be gone. How can I recover it?

You can restore a grid using the history found in Extenso.
In the right column there is a Section called "Grid Recovery".

From there, you can see the generation history of the grid and go back by pressing the desired date and time. This will regenerate the page automatically.

Note:
The restoration is only for the design of the grid and not the content of each of the widgets. For example, it is not possible to recover an old text which would have been modified.