Unable to access file <unspecified filename> since it is locked
When attempting to power on a virtual machine using the VI-Client you receive the error "Unable to access file <unspecified filename> since it is locked"
I haven't found a specific cause for this issue but something happens within ESX where it can no longer manager the process for the virtual machine in question. Can't power on, off, reset, etc. Removing from and adding the device back into the inventory have no effect. I have seen this issue in ESX 3.5 and vSphere 4.0
Connect to each ESX host that the VM may have been running on use "ps -ef" to view the processes running on that host. "grep" for the name of the VM in question. When you find the process for that VM, kill it ("kill -9 PID"). Be aware that if that VM was running, it will no longer be after you kill it's host process. Now you should be able to power on and manage this VM again.