I'm using Sharepoint Online for a client and I have very little understanding of how Sharepoint works. I apparently have completely broken the site. After two days of hard work, most of the site works again, but some pages are still unavailable: lots of configuration pages just crash when I access them. It's impossible to change the site's theme, it's impossible to use the 'Add page' feature, the homepage is broken and can't be set/fixed... it's pretty catastrophic.
The sanity check from the Sharepoint settings page shows all green, but all these problems are still happening.
These issues are happening on the site that is at the root of the site collection, so I don't have the option of deleting the site. I need a way to reset it from scratch. I do not care if it ends up destroying the site subsites as well. If I could just go back to a 'blank' account like it was the first time I accessed Sharepoint Online, it'd be perfect. If I can keep the subsites though, it'd be even better.