How to solve Fatal error: Uncaught exception 'PDOException' with message 'SQLSTATE[HY000]: General error: 2006 MySQL server has gone in Drupal 7?

This issue will come generally because of allocation of memory in mysql, to over come this just run the below query in mysql server.

mysql> SET max_allowed_packet = 1024 * 1024 * 512;

 or  

mysql> SET global max_allowed_packet=1024 * 1024 * 512;