@carlo-laszlo , this might be a question for support. In the meantime, however…
First, have you tried closing out your session and restarting it, just to see if this is a glitch that could be fixed by a restart?
Also, a question. When you say the grid is gone, do you mean that when you select the section, you no longer have the option to ‘edit’ the grid, only to ‘apply’ a grid?
If that’s the case, have you tried stepping back to a prior version in Site History? It might be that a very recent version – one that wouldn’t require a lot of re-work – still has the grid.
You might also try re-applying the grid. I just experimented with that on a test section, and it was pretty messy – realigning items to the new grid can impact the mobile version – so that would definitely not be my first choice.