Your logs show a normal shutdown (Signal 15)
It does not show that it came back up after that.
Did you enable PMS to autostart at system boot?
ep 14, 2018 19:51:31.920 [0x7f6b2afff700] DEBUG - Completed: [127.0.0.1:59244] 200 GET /:/timeline?ratingKey=9388&key=%2Flibrary%2Fmetadata%2F9388&playQueueItemID=66107&state=paused&hasMDE=1&time=66000&duration=3031000 (8 live) GZIP 16ms 490 bytes (pipelined: 4)
Sep 14, 2018 19:51:32.634 [0x7f6b3aab7800] DEBUG - Shutting down with signal 15
Sep 14, 2018 19:51:32.634 [0x7f6b3aab7800] DEBUG - Ordered to stop server.
Sep 14, 2018 19:51:32.635 [0x7f6b257ff700] DEBUG - Jobs: '/usr/lib/plexmediaserver/Plex DLNA Server' exit code for process 1679 is -15 (signal: Terminated)
Sep 14, 2018 19:51:32.635 [0x7f6b0abff700] DEBUG - Plex DLNA Server exited.
Sep 14, 2018 19:51:32.636 [0x7f6b257ff700] DEBUG - Jobs: '/usr/lib/plexmediaserver/Plex Media Scanner' exit code for process 18319 is -15 (signal: Terminated)
Sep 14, 2018 19:51:32.638 [0x7f6b3aab7800] WARN - Timed out waiting for server to finish.
Sep 14, 2018 19:51:32.640 [0x7f6b257ff700] DEBUG - Collections: Found 2 collections tags in section 1.
Sep 14, 2018 19:51:32.640 [0x7f6b257ff700] DEBUG - Collections: We had 2 collections in the library already.
Sep 14, 2018 19:51:32.640 [0x7f6b257ff700] DEBUG - Collections: Synced tags in section 1 in 0ms.
Sep 14, 2018 19:51:32.647 [0x7f6b3aab7800] DEBUG - Stopping server...
Sep 14, 2018 19:51:32.647 [0x7f6b3aab7800] DEBUG - HttpServer: Stopping server.
Sep 14, 2018 19:51:32.648 [0x7f6b1e3ff700] DEBUG - MetadataItemClusterRequestHandler: a shutdown event has been received
Sep 14, 2018 19:51:32.648 [0x7f6b1e3ff700] DEBUG - TranscodeJobManager: [Running] onServerShutdown
Sep 14, 2018 19:51:32.648 [0x7f6b1e3ff700] DEBUG - TranscodeJobManager: [Running] -> [Shutdown]
Sep 14, 2018 19:51:32.650 [0x7f6b3aab7800] DEBUG - Server stopped.
Sep 14, 2018 19:51:32.652 [0x7f6b257ff700] DEBUG - Collections: Refreshed attributes for 2 collections in section 1 in 11ms.
Sep 14, 2018 19:51:32.654 [0x7f6b257ff700] DEBUG - Collection: Refreshed visibility in section 1 in 2ms.
Sep 14, 2018 19:51:32.655 [0x7f6b3aab7800] DEBUG - Server is already in shutdown mode, ignoring this signal...
Sep 14, 2018 19:51:32.655 [0x7f6b3aab7800] DEBUG - NetworkService: Stopping advertisement.
Sep 14, 2018 19:51:32.655 [0x7f6b267ff700] DEBUG - Activity: registered new activity 4a83add1-64c7-4375-91ed-54369a00dc83 - Processing subscriptions
Sep 14, 2018 19:51:32.655 [0x7f6b213ff700] ERROR - Network Service: Error in advertiser handle read: 125 (Operation canceled) socket=-1
Sep 14, 2018 19:51:32.657 [0x7f6b133fd700] WARN - SLOW QUERY: It took 352070.000000 ms to retrieve 0 items.
Sep 14, 2018 19:51:32.660 [0x7f6b133fd700] DEBUG - Activity: registered new activity 88136e9b-c926-47ce-89d1-d0a66814994c - Processing subscriptions
Sep 14, 2018 19:51:32.660 [0x7f6b133fd700] DEBUG - DVR:Subscription: Scheduling subscriptions.
Sep 14, 2018 19:51:32.662 [0x7f6b133fd700] DEBUG - DVR:Subscription: Initialized 0 schedulers.
Sep 14, 2018 19:51:32.662 [0x7f6b133fd700] DEBUG - DVR:Subscription: Starting with 0 grabs, scheduled 0 active ones preemptively.
Sep 14, 2018 19:51:32.663 [0x7f6b133fd700] DEBUG - DVR:Subscription: Scheduled all subscriptions in 2 ms.
Sep 14, 2018 19:51:32.663 [0x7f6b133fd700] DEBUG - Activity: Ended activity 88136e9b-c926-47ce-89d1-d0a66814994c.
Sep 14, 2018 19:51:32.663 [0x7f6b133fd700] DEBUG - DVR:Subscription: There are 0 active grabs at the end.
Sep 14, 2018 19:51:32.663 [0x7f6b133fd700] DEBUG - Activity: Ended activity 4a83add1-64c7-4375-91ed-54369a00dc83.
Sep 14, 2018 19:51:32.663 [0x7f6b133fd700] DEBUG - DVR:Subscription: Refreshed all content in 7 ms.
Sep 14, 2018 19:51:32.663 [0x7f6b257ff700] DEBUG - Jobs: '/usr/lib/plexmediaserver/Plex Media Scanner' exit code for process 32652 is -15 (signal: Terminated)
Sep 14, 2018 19:51:32.665 [0x7f6b3aab7800] DEBUG - Killing plug-ins.
Sep 14, 2018 19:51:32.665 [0x7f6b3aab7800] DEBUG - [com.plexapp.system] Killing.
Sep 14, 2018 19:51:32.665 [0x7f6b3aab7800] DEBUG - Waiting for Plex DLNA Server to exit.
Sep 14, 2018 19:51:32.727 [0x7f6b257ff700] DEBUG - Jobs: '/usr/lib/plexmediaserver/Plex Tuner Service' exit code for process 3022 is 0 (success)
Sep 14, 2018 19:51:32.727 [0x7f6b1f3ff700] DEBUG - Plex Tuner Service exited.
Sep 14, 2018 19:51:32.735 [0x7f6b3aab7800] DEBUG - Waiting for Plex Tuner Service to exit.
Sep 14, 2018 19:51:32.755 [0x7f6b213ff700] INFO - Network Service: Abandoning advertise socket, it was closed.
Sep 14, 2018 19:51:32.829 [0x7f6b3aab7800] DEBUG - Killing job.
Sep 14, 2018 19:51:32.829 [0x7f6b3aab7800] DEBUG - Signalling job ID 18319 with 9
Sep 14, 2018 19:51:32.829 [0x7f6b3aab7800] DEBUG - Database: Shutting down.
Sep 14, 2018 19:51:32.829 [0x7f6b3aab7800] DEBUG - Captured session 0.
Sep 14, 2018 19:51:32.829 [0x7f6b3aab7800] DEBUG - Captured session 1.
Sep 14, 2018 19:51:32.829 [0x7f6b3aab7800] DEBUG - Captured session 2.
Sep 14, 2018 19:51:32.829 [0x7f6b3aab7800] DEBUG - Captured session 3.
Sep 14, 2018 19:51:32.829 [0x7f6b3aab7800] DEBUG - Captured session 4.
Sep 14, 2018 19:51:32.829 [0x7f6b3aab7800] DEBUG - Captured session 5.
Sep 14, 2018 19:51:32.829 [0x7f6b3aab7800] DEBUG - Captured session 6.
Sep 14, 2018 19:51:32.829 [0x7f6b3aab7800] DEBUG - Captured session 7.
Sep 14, 2018 19:51:32.829 [0x7f6b3aab7800] DEBUG - Captured session 8.
Sep 14, 2018 19:51:32.829 [0x7f6b3aab7800] DEBUG - Captured session 9.
Sep 14, 2018 19:51:32.829 [0x7f6b3aab7800] DEBUG - Captured session 10.
Sep 14, 2018 19:51:32.829 [0x7f6b3aab7800] DEBUG - Captured session 11.
Sep 14, 2018 19:51:32.829 [0x7f6b3aab7800] DEBUG - Captured session 12.
Sep 14, 2018 19:51:32.829 [0x7f6b3aab7800] DEBUG - Captured session 13.
Sep 14, 2018 19:51:32.829 [0x7f6b3aab7800] DEBUG - Captured session 14.
Sep 14, 2018 19:51:32.829 [0x7f6b3aab7800] DEBUG - Captured session 15.
Sep 14, 2018 19:51:32.829 [0x7f6b3aab7800] DEBUG - Captured session 16.
Sep 14, 2018 19:51:32.829 [0x7f6b3aab7800] DEBUG - Captured session 17.
Sep 14, 2018 19:51:32.829 [0x7f6b3aab7800] DEBUG - Captured session 18.
Sep 14, 2018 19:51:32.829 [0x7f6b3aab7800] DEBUG - Captured session 19.
Sep 14, 2018 19:51:32.846 [0x7f6b3aab7800] DEBUG - Database: Shutting down.
Sep 14, 2018 19:51:32.846 [0x7f6b3aab7800] DEBUG - Captured session 0.
Sep 14, 2018 19:51:32.846 [0x7f6b3aab7800] DEBUG - Captured session 1.