There are plenty of possible reasons, but if you make some change in your
/etc/my.cnf before rebooting your server, the one strong possibility is that it doesn’t like something in your new config in
Also if MySQL is not starting after an upgrade to newer version, be sure that somethings in your
/etc/my.cnf were depreciated and cause newer versions of MySQL to choke on startup. so try to check every line is it compatible with the new version.
For both raison above, the quick-fix in this case is to simply rename your existing
/etc/my.cnf file, and try restarting MySQL again (
/etc/init.d/mysql restart should do the trick). If it works, create a new my.cnf (or copy over one of the defaults included with the installation) and start re-adding your previous tweaks, being sure to restart MySQL after each change. Eventually you’ll probably hit the one that made MySQL choke, and you can simply omit it at that point.