Couldn't start mysql server. It only worked after reboot.

Hi, here’s what happened Yesterday around 9am (London time):

150217 09:04:29 mysqld_safe Starting mysqld daemon with databases from /var/lib/mysql 150217 9:04:29 [Warning] Using unique option prefix key_buffer instead of key_buffer_size is deprecated and will be removed in a future release. Please use the full name instead. 150217 9:04:29 [Warning] Using unique option prefix myisam-recover instead of myisam-recover-options is deprecated and will be removed in a future release. Please use the full name instead. 150217 9:04:29 [Note] Plugin ‘FEDERATED’ is disabled. 150217 9:04:29 InnoDB: The InnoDB memory heap is disabled 150217 9:04:29 InnoDB: Mutexes and rw_locks use GCC atomic builtins 150217 9:04:29 InnoDB: Compressed tables use zlib 1.2.8 150217 9:04:29 InnoDB: Using Linux native AIO 150217 9:04:29 InnoDB: Initializing buffer pool, size = 128.0M InnoDB: mmap(137363456 bytes) failed; errno 12 150217 9:04:29 InnoDB: Completed initialization of buffer pool 150217 9:04:29 InnoDB: Fatal error: cannot allocate memory for the buffer pool 150217 9:04:29 [ERROR] Plugin ‘InnoDB’ init function returned error. 150217 9:04:29 [ERROR] Plugin ‘InnoDB’ registration as a STORAGE ENGINE failed. 150217 9:04:29 [ERROR] Unknown/unsupported storage engine: InnoDB 150217 9:04:29 [ERROR] Aborting

150217 9:04:29 [Note] /usr/sbin/mysqld: Shutdown complete

150217 09:04:29 mysqld_safe mysqld from pid file /var/run/mysqld/mysqld.pid ended

Does anyone know why this could happen and what would be the fix for it (other than reboot)?

Thanks, Lukasz