If you have several VMs in your VMware inventory that are shown as either inaccessible or invalid, you can actually reload the configuration without the need to shutdown the VM or put the host in maintenance mode.

This is done via the reload method on the Virtual Machine View object in PowerCLI. The result is effectively the same as dropping the VM from the inventory and adding it back via the vmx file. 

Get-View -ViewType VirtualMachine | ?{$_.Runtime.ConnectionState -eq "invalid" -or $_.Runtime.ConnectionState -eq "inaccessible"} | % {$_.reload()}

Ensure you are in the PowerCLI and connection to your vCenter server before executing this command or it will fail.