Is mysqld running on your system? Try restarting it? If you have one / filesystem I'd use "du -sh /" to look which directory uses most space and work from there, or if /var is seperate filesystem and it's full, "du -sh /var" and then dig down to see what could be deleted.
Look at /var/log/messages, /var/log/syslog, /var/log/daemon or something similar if mysqld refuses to start.
Br,
Hannu