Why can’t I edit a page?
Someone on your team may have checked out the page so it’s locked. To edit the page, you need to force a check in. Go to Site Contents > Pages folder. Find the page. Click on the 3 dots to the right of it, until you can get the Check In option.
Can I revert back to an older version of a page?
Yes, go to Page > Page History. Find a version and hit Restore.
I deleted a file. How can I get it back?
SharePoint keeps deleted files in the Recycle Bin. It will be emptied every 30 days. You would need to be a site collection administrator to access the Recycle Bin. Go to Site Settings > Site Collection Administration > Recycle bin. Check the items you want to get back. Click Restore.