Hi there!
what have you tried?, in this post I’ve put many different troubleshooting steps so it would be helpful to know what you’ve tried already so we know what can be discarded.
Just to sum up, this is what happens usually (I hope I’m right 
)
- If the VM machine is running…
- The VNC button in Sunstone tries to open a connection against 29876/tcp port where the noVNC proxy is listening.
- Then VNC proxy in the frontend tries to open a connection against 59XX port on the node/hypervisor where XX is the ID of the VM.
Are you using KVM as hypervisor or vCenter?
Are you using Apache or NGINX in front of Sunstone? Using SSL/TLS?
Cheers!