I had the same problem a few days ago after crashing an Alternc installation. The solution is to check your logs under /var/log/syslog to see that there is no write access to temp folder /tmp. Just do a "chmod 777 /tmp" and restart mysql "/etc/init.d/mysql restart".

But with such kind of an error i guess you have a big trouble on your server, think for a complete reinstallation...