Microsoft has posted a service advisory for SharePoint Online and Onedrive between 10pm on Thursday, June 14th and 2am on Friday, June 15th. During this time, SharePoint and One Drive for Business content will typically be read-only for 30-60 minutes. However, it is possible that in some rare cases, the read only window may extend for up to 4 hours.
Microsoft has scheduled this outside of normal hours to minimize the impact. If you have any questions, please contact the Help Center at x58888 or firstname.lastname@example.org