Has anything on the system recently been restored from backup? Was _internal disabled manually or by Splunk itself? You can look for something like this in splunkd.log that can help tell you where the problem is:
06-20-2019 23:06:05.513 +0000 ERROR IndexProcessor - One or more indexes could not be initialized and were automatically disabled, please see splunkd.log for more details
06-20-2019 23:06:05.512 +0000 ERROR IndexProcessor - caught exception for index=_internal during initialzation: 'Splunk has detected that a directory has been manually copied into its database, causing id conflicts [/opt/splunk/var/lib/splunk/_internaldb/db/hot_v1_1, /opt/splunk/var/lib/splunk/_internaldb/db/db_1307563383_1307490866_1].'.Disabling the index, please fix-up and run splunk enable index
If this is the case, Splunk would need to be stopped, bucket collisions remedied, indexes reenabled, and then restarted. A good answer on this can be found here: https://answers.splunk.com/answers/30986/why-is-my-index-disabled.html.
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