Error 503 Backend fetch failed: "obj_theme"?

JKP / 2016-07-11 16:55:35   

After years of faithful uneventful service my site has gone down just giving a 503 error:

Error 503 Backend fetch failed
Backend fetch failed
Guru Meditation:
XID: 245784712
Varnish cache server

I have tried uploading a backup of the site with no change and have been in touch with my host (Easyspace) who are now just referring me back to my developer..

"I have checked your issue in detail and unable to find the exact issue, I have tried to debug the error in display, but it wont show anything in the debug.
I suspect the issue with the plugin or the theme "obj_theme". Could you please contact the developer regarding this issue? "

I have spent some time looking through the forum archive but only one post relating to this 503 error which didn't help and nothing on Varnish.

Any ideas what to look at next?

This thread has been closed, thank you.