Problem Statement: The Magento and WordPress websites hosted on Linux server suddenly stopped. Upon checking MySQL error log following error message traced out:

Plugin 'InnoDB' init function returned error
Plugin 'InnoDB' registration as a STORAGE ENGINE failed.

Cause: One of the common possible causes is my.cnf file has been modified and saved with incorrect structure.

Solution:
The innodb_log_file_size might not have actual size of the existing InnoDB log files.
Step-1: login into the server via SSH with root access
Step-2: go to /var/lib/mysql
Step-3: Rename or move both files ib_logfile0 and ib_logfile1 files
Step-4: Stop and start the MySQL service (Do not interrupt MySQL service execution because it may take some time to start)

Notes:
[1] Location of My.cnf in Linux: /etc/my.cnf
[2] What is Innodb?
InnoDB is a storage engine for MySQL, included as standard in all current binaries distributed by MySQL AB. Its main enhancement over other storage engines available for use with MySQL is ACID-compliant transaction support