I've been running a non-commercial WordPress blog since 2009. Yesterday, I tried to create a new blog post but there was an issue with the database and now the WP Dashboard tells me that there are no posts, no activity, no comments, etc. Jetpack Stats can't find anything to show. The public interface now shows a "Page not found" with the message "Sorry, no posts matched your criteria".

I contacted my ISP's tech support and after a long back-and-forth, they reported:

"Upon reviewing logs, It shows Could not insert post into the database while adding post in wordpress dashboard, which typically occurs due to unsupported character encoding (like emojis), corrupted database tables, an omitted AUTO_INCREMENT attribute, or insufficient server memory limits.

"I have successfully repaired and optimize database am afraid issue remains same"

They ran a malware scan which found four instances of:

SL-PHP-BACKDOOR-GENERIC-hnl.UNOFFICIAL 

As a user, how do I remove these four instances?