I faced a quite interesting situation today where I wanted to put my SharePoint side down, I mean I wanted to stop users from accessing it apart from the site collection admin. Removing User's permission was a straight forward approach but my site had 100+ groups and I'll need to reconfigure them again after the downtime.
So I wanted to know if there's anyway to make a SharePoint Unavailable for a specific period. I am currently using Office 365 though tips for SharePoint 2013 on premise would also be appreciated.