Are you running Debian Squeeze?
Did you upgrade to mysql-server-5.1 ?
Are you getting a message like this?
Starting MySQL database server: mysqld . . . . . . . . . . . . . . failed!
Did you read /usr/share/doc/mysql-server-5.1/README.Debian.gz ?
Go read it again and this time *comment out* skip-bdb in /etc/mysql/my.cnf:
perl -i.bak -pe 's/^(skip-bdb)/#$1/' /etc/mysql/my.cnf