I have been having a similar issue as well. All of my collections and collection pages are completely gone from the Designer and when I first login an error message pops up saying “Update Page Slug, Slugs to update: undefined Template”. I never made any pages by that name though so I have to assume its related to the problem - you can see it in the screenshot below.
I have been experiencing this problem since Tuesday afternoon and reached out to support yesterday morning but have not received a reply. The majority of the content on my site is controlled by Collections and I currently can’t make any changes at all in fear that publishing it will affect the live site as well.
I know that there was a new CMS update announced yesterday stating that you can now add files to collection items - is it possible that this broke some sites’ existing Collections? Will anything change if I revert back to an older version of the site just before things broke?
My sites link is https://www.defelsko.com/ and the read-only link is here. I am using Google Chrome Version 68.0.3440.106 on macOS Sierra Version 10.12.6.