And it happened again a few minutes ago.
That must be some funny bug in SP Page Builder i think.
I found a backup of files on server and restore them.
I only changed row options and started visually changing padding (as shown on picture).
And bang - suddenly everything on backend disappeared except menu and footer.
Undo option did not changed anything, luckly I did not saved changes.
I closed SP Pagebuilder, run it again and everything is as before.
I can learn to live with it in that project, but maybe it can help you a little to find the reason.