ESXi Standalone Host Cannot Power ON VM's

We have a standalone Vmware ESXi host on ESXi 5.1. It currently has a handful of VMs powered on, and they are running fine. If I try to power on any other VM - any VM at all -Also any one of the machine Power off and not able to start it again. I am receiving the following message:

Solution :- Restart of the management agents.

                Gain SSH access to your standalone host and run: /sbin/services.sh restart