First try that your hostname is able to resolve ipaddress.
eg:- Ping SplunkServer Hostname.
If it is successfull
than try to telnet your splunkserver hostname with port 8000
If you are not able to telnet port 8000, than go to splunk server. if it is Window Server than restart the splunk services in services.msc
If it is Linux server than restart the service with (sudo 'path of your splunk' start)
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