I didn't stop mongoDB. Did it crash? This looks like something stopped/killed it intentionally.
[root@hercules mongodb]# systemctl status mongod
mongod.service - SYSV: Mongo is a scalable, document-oriented database.
Loaded: loaded (/etc/rc.d/init.d/mongod)
Active: failed (Result: signal) since Sun 2015-05-17 23:50:38 CDT; 3h 14min ago
Main PID: 3388 (code=killed, signal=KILL)
CGroup: /system.slice/mongod.service
May 03 19:36:29 hercules runuser[3254]: pam_unix(runuser:session): session opened for user mongod by (uid=0)
May 03 19:36:31 hercules runuser[3254]: pam_unix(runuser:session): session closed for user mongod
May 03 19:36:31 hercules mongod[3226]: Starting mongod: [ OK ]
May 03 19:36:31 hercules systemd[1]: Started SYSV: Mongo is a scalable, document-oriented database..
May 17 23:50:37 hercules systemd[1]: mongod.service: main process exited, code=killed, status=9/KILL
May 17 23:50:38 hercules mongod[11702]: Stopping mongod: [ OK ]
May 17 23:50:38 hercules systemd[1]: Unit mongod.service entered failed state.