We use Sydney theme and WordPress 4.7.3 on www.nyaramstiftelsen.se
I was editing Dashboard > Appearance > Customize > Additional CSS, trying to make the main menu wider, to avoid line a break. I was trying some of the CSS code suggestions given in this support forum. At one point I deleted the code I previoulsy added as the menu did not get wider, and also the help text in the Additional CSS window, and saved. I should not have deleted the help text, as the menu Additional CSS disappeared from Customize menu.
I asked One.com support and they advised that I restore a backup of webcontent and database. I did this from a couple of days earlier. However the problem is still there. Would you know if and how we can get the menu back?