You experience a sudden site lock "read-only", what appeared to be a total lock down on the SharePoint sites. It results in ALL users, even site administrators and users with full administrative rights are unable to do anything! Nobody could add new documents or edit documents; nearly every single Document or Library option is greyed out!
A possible cause could be that you started a SharePoint backup (during backup the site is locked) via Powershell and canceled the process while running. So the command is not able to unlock the sites afterwards.
Solution:
Fix via the central administration website:
- Central Administration -> Site Collection Quotas and Locks.
- Select your site collection
- Change the Site Lock Information radio button from “Read-only” to “Not locked”
- OK
