Hello,
This post was moved to the VMware vCenter Server forum.
This sometimes happens when the vCenter Server 'service' is no longer running, which can happen on occasion. This would be the first thing you should check. It could also imply that there are some outgoing/incoming ports blocked somewhere as well.
Generally, you should only access vCenter through a jump machine within the same security zone. Use RDP to access the jump machine is usually best. This way all access is controlled and you do not need to worry about pesky firewall rules.
Best regards,
Edward L. Haletky
VMware Communities User Moderator, VMware vExpert 2009, 2010, 2011,2012,2013,2014
Author of the books 'VMWare ESX and ESXi in the Enterprise: Planning Deployment Virtualization Servers', Copyright 2011 Pearson Education. 'VMware vSphere and Virtual Infrastructure Security: Securing the Virtual Environment', Copyright 2009 Pearson Education.
Virtualization and Cloud Security Analyst: The Virtualization Practice, LLC -- vSphere Upgrade Saga -- Virtualization Security Round Table Podcast