I just had something like this happen to a site in progress that has not been launched yet. Our shop page changed to a blog page, lost a lot of content and was ineditable. Was raking my brain to think what happened and started reworking the page but since editing was not working, decided best to contact SS. SS restored the page and content rather quickly, but I am leery and worried about this issue. This was a very strange and random "bug". SS's response was not completely reassuring:
"This was actually triggered by an internal bug within our platform, that has been identified by our engineers and they are working to solve this ASAP. However, they haven't found the pattern that triggers this behaviour. Having said that, the large majority of user reports, point to the fact that, this only occurs once on a Squarespace website, so the possibility to occur again within your website is limited. "