The problem we are experiencing is very new to us. We did not experience this ever before with our website or with our web server (in last 8 years). Just recently for last 2 week we have this issue and it happened over 3 times.
Our site is experiencing "White Screen of Death". We do not see any error. Please see screenshot for better understanding. its just that. The site just tries to load but cannot.
Findings : what we have done to fix this
We can fix it (temporarily) if we reboot the server (restarting the whole virtual machine).
But this is not a viable solution . As we have other Wordpress and non-Wordpress websites in that server which does not have that issue. And rebooting the server means all sites in the server are down for 5-10 minutes. After rebooting the server site comes back and works fine as usual.
But This problem occurred at least 3 times since last week. We cannot replicate the error by clicking on any page. There is no particular time of the day when this happens.
The only significant change we can think of is upgrading the PHP version for the site from 5.3 to 7.2 in our web server. FYI we have upgraded the PHP version last month. And for approximately 3 weeks we have not seen this or any other issue. The wordpress version of the site is 4.9.1
Webserver - Apache
Database - My SQL 5.1.7
We have plenty free disk space in the server
We have tried to look in to the error log at the time of the problem . but we could not find any thing relevant. This is the error we could see on error log. Although I didn't think it was the issue but I have corrected the SQL query mentioned in the error log. But we have faced the same issue again today.
I can see the following log files from my cpanel
[Tue Apr 23 13:48:35 2019] [error] [client 126.96.36.199] FastCGI: server "/var/run/psychicf-remi-safe-php72.fcgi" stderr: PHP message: WordPress database error You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'ORDER BY number DESC' at line 1 for query SELECT * FROM view_site_numbers WHERE campaign_id = "7" AND `use` = "PPC" LIMIT 2 ORDER BY number DESC made by require('wp-blog-header.php'), require_once('wp-includes/template-loader.php'), include('/themes/psychicFuture/css/content-all-horoscopes.php'), get_header, locate_template, load_template, require_once('/themes/psychicFuture/header.php'), do_shortcode, preg_replace_callback, do_shortcode_tag, get_prs_number_register, referer: https://thehoroscopejunkie.ca/weekly_horoscopes.html