If the mysql server is not behaving normally, you might have some corrupted tables. It happens usually after an unscheduled restart. mysqldump won't be able to export corrupted tables.
If the mysql server is running, try connecting to it using phpmyadmin and use the built in tools to fix and check the tables
if the mysql server fails to start, check the log files and use the mysql command line tools to fix the issues
myisamchk --silent --force --fast --update-state /var/lib/mysql/dbname/*.MYI
(http://www.thegeekstuff.com/2008/09/how-to-repair-corrupted-mysql-tables-using-myisamchk/)