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If you refer to your logs (and have sufficient logging configured) you'll instantly find the problem.
Also, be sure you're running mysqltuner on a regular basis + working to quiet all diagnostics produced.
Big Fix: If you really are using MySQL, then remove MySQL software packages (leaving /var/lib/mysql data in place), then install latest MariaDB packages from the official repositories.
Many Distros have/are dropping MySQL completely + installing MariaDB packages when MySQL package targets are installed.
Many MySQL versions, especially older versions, have all sorts of bugs. Use MariaDB instead, to ensure you're starting with stable/working database code.