I manage a wordpress blog for my company and we experienced this same issue. It turned out that the server had run out of space when one of our writers was writing a draft and saving it. It had corrupted the ‘wp_posts’ table in the MySQL db. I ran a repair on that table and everything came back. Here is a link to a howto to repair a table in MySQL. https://www.karakas-online.de/EN-Book/repair-corrupt-table.html
I hope this helps.
