Solution:
Sounds like a curupted wordpress update. If your worried about losing data just preform a rollback. You can have your website back online in 20 minutes or less.
Basically all you do is:
1: Download previous version of WP 2: For now move your wp admin and wp includes in temp directory (name it whatever). Also include all other files by wordpress not in these directories (wp config.php, etc).
So you should have wp-content and tmp directory only (cgi-bin is fine)
- Upload previous wordpress zip file and unzip.
- Assuming this version unzips with one wordpress directory simply delete wp-content directory from this version. So if you haven’t moved anything then /wordpress/
- Move all content from this wordpress folder out (wp content should be deleted).
- If you had custom httaccess simply move your old one from tmp folder.
- Now go into wp-config.php and add your credentials for sql db.
- I would clear cache and test wordpress. If everything works then delete your temp directory that you created that has 5.1.1 within in addition to your empty wordpress directory.
If you are using Cloudflare also purge all cache.
All your data is stored in wp-content and SQL db.
No data will be lost so everything should perform as it did before the update occured.
Hope this helps, let me know if you encounter any issues.