mysql55 spontaneous malfunction
Did you see this in the log file?


160930  3:05:54 InnoDB: Fatal error: cannot allocate memory for the buffer pool


You do not have enough memory available on your host. Either decrease the amount of memory MySQL is configured to use, or increase the amount of memory the host/VM has.

The database cannot start because there is not enough memory. 

Speculating as to why the database restarted in the first place, it may well have been that the operating systems OOM Killer chose the MySQL process as "the one" when the OS was exhausted of memory. See How OOM killer decides which process to kill first? on Unix.SE. 

The "Unknown/unsupported storage engine" message appears because MySQL has asked the storage engine (InnoDB) to initialize itself, which it cannot do due to lack of memory. 


160930  3:05:54 [ERROR] Plugin 'InnoDB' init function returned error.
160930  3:05:54 [ERROR] Plugin 'InnoDB' registration as a STORAGE ENGINE failed.
160930  3:05:54 [ERROR] Unknown/unsupported storage engine: InnoDB