VMware virtual machines grayed out in Virtual Center

It happened to me already twice, with no apparent cause. Fortunately, the resolution is very straightforward, you just need to restart the management agents on ESX server.

Here is the link to VMware site:

http://kb.vmware.com/selfservice/microsites/search.do?language=en_US&cmd=displayKC&externalId=1003490

Here is the contents of it, so you can follow the procedure easily:

Restarting the Management agents on ESX Server 3.x

To restart the management agents on ESX Server 3.x:

Login to your ESX Server as root from either an SSH session or directly from the console of the server.
Type service mgmt-vmware restart .

Caution: Ensure Automatic Startup/Shutdown of virtual machines is disabled before running this command or you risk rebooting the virtual machines. For more information, see Restarting hostd (mgmt-vmware) on ESX Server Hosts Restarts Hosted Virtual Machines Where Virtual Machine Startup/Shutdown is Enabled (1003312).
Press Enter.
Type service vmware-vpxa restart .
Press Enter.
Type logout and press Enter to disconnect from the ESX Server.

If this process is successful, it appears as:

[root@server]# service mgmt-vmware restart
Stopping VMware ESX Server Management services:
VMware ESX Server Host Agent Watchdog [ OK ]
VMware ESX Server Host Agent [ OK ]
Starting VMware ESX Server Management services:
VMware ESX Server Host Agent (background) [ OK ]
Availability report startup (background) [ OK ]
[root@server]# service vmware-vpxa restart
Stopping vmware-vpxa: [ OK ]
Starting vmware-vpxa: [ OK ]
[root@server]#