1286 - Unknown storage engine 'InnoDB'
OBSERVATIONS


I see there is nothing Innodb-related in the my.cnf.
Look for Innodb-related error messages expressing why mysqld failed to start in the error log.


My first guess would be look at the ib_logfile0 and ib_logfile1 in /var/lib/mysql.

If they have been set to size different from the default, that probably will prevent mysqld's startup. The default for innodb_log_file_size is 5M.

SUGGESTION #1

If RoundCube's installation overwrote the my.cnf, see if you have a physical backup of the previous my.cnf. Put that back in place and run

service mysql start


SUGGESTION #2

Take the filesize of ib_logfile0 and divide by 1048576. Let's say it is 12M.

Put that in my.cnf

[mysqld]
innodb_log_file_size = 12M


then run

service mysql start


Give it a Try !!!