I saw this error while troubleshooting another problem on squid (DNS issue)
[root@pswproxy etc] # service squid status
squid (pid 3070 3068) is running...
squid: ERROR: No running copy
This is normally due to squid.pid file missing, since this file is present whenever squid is running, if this file is missing squid will not work. If it is deleted by mistake so still squid will be in running condition and will not work properly, so I found this article very useful. In my case I just restart the squid services and it starts working.
I hope this will be informative for you!