We have this happen on our old CS-Cart site fairly often and after going through Cpanel raw access logs I found that we had been having bot attacks via multiple proxy servers and it crashes MySQL which then needs to be rebooted, as mentioned by ecomlabs.
There are no shortage of these black hat fools creating malicious bots and they can cause a site to go down unexpectedly so it may be worth looking at your raw site logs just prior to the time of that your site went down to see if you have lots of hits coming from a few IP’s.
If so then you can use “IP Blocker” in cPanel to block the specific IP’s (or better yet the entire IP range), if you do a WHOIS search you can establish what the source of the IP is, most often they use proxies so they will move on to the next proxy anyway. They also use TOR so it is worth IP banning the entire TOR node list and the TOR exit list (the off ramp for onion router).
The other thing you can do is if you have access to WHM panel or if your hosting is helpful you can use CPHulk to block specific IP’s IP ranges and geographical regions, black hat fools tend to be creatures of habit so they often have a preference for using Proxies that are from specific countries which make them feel warm and safe while hiding in their mums basement.
Thank you for suggestions.
My website is down now for more than 48 hours.
The story started weird. I was logged in as admin and worked on products. And came up error Service not available. I have checked an error and error was related to @MAURISWEB addon manager. This addon was installed more than week ago. Were no issues. I am not blaming on addon, just telling facts how started problem. So, front end and back end Service not available. I have restarted server and was able to login to back end. Straight away I have disabled that addon. After minute again error Service not available to back end and front end. The error posted above. Also I cannot access Plesk. The restart of server not helps anymore.
Good day! If there are errors and you think that a failure occurred in the operation of our module, please email us a description of the error (if possible) and provide access to FTP, mysql and the admin panel of your site for analysis.
We once faced a situation when a client installed modules and worked for a long time. After the release of our update, I updated the modules, and the provider crashed and started automatic recovery of the database server created by mysql the day before. As a result, there were files on the server, but there were no entries in mysql.
In any case, we need access to the server, Database and the admin panel of the site to find the reason.