Details you can only get through UI or API , considering web endpoint is reachable, Seems in this case NiFi service is up but UI is not available or NiFi service is completely down , can be confirm by checking its process status. So you need find way to bring back the NIFI service along with UI in minimalistic load by stopping all flows by setting nifi.flowcontroller.autoResumeState to false and start the service, by doing this nifi will start with all the queued data but it will not do any processing, this can give you the UI but since you have said that are too much data so startup will take time. 

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Thank you,

Chandan