If you upgrade from MySQL 5.0 to MySQL 5.5 through WHM / cPanel, you may find that MySQL doesn’t start afterwards. It’ll dump out a message like:
ERROR! MySQL server PID file could not be found!
UPDATE: cPanel put out a fix on Oct 12 2011, in the latest WHM EDGE (version 220.127.116.11). Mind you, it may not help if you’re already in this situation (so keep reading), but should help prevent it from happening to others – from the looks of it the depreciated options will be handled during the upgrade, assuming you’ve upgraded WHM to beyond 18.104.22.168 before upgrading MySQL to 5.5 anyway.
There are plenty of possible reasons, but one strong possibility is that it doesn’t like something in your old my.cnf file. Some things in 5.0 were depreciated and cause newer versions of MySQL to choke on startup (I found this out playing with 5.1 on another server, and it again hit me again recently).
The quick-fix in this case is to simply rename your existing my.cnf file (located in /etc/my.cnf ), and try restarting MySQL again ( /etc/rc.d/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.