VirtualBox 4.3.4 VT-x error.

Hi, today when I start Windows in VirtualBox it display this error

Failed to open a session for the virtual machine W7.  VT-x is not available. (VERR_VMX_NO_VMX).  Result Code: NS_ERROR_FAILURE (0x80004005) Component: Console Interface: IConsole {8ab7c520-2442-4b66-8d74-4ff1e195d2b6}

I google this error and found this post on VirtualBox forum that solved the problem.
      https://forums.virtualbox.org/viewtopic.php?f=7&t=58826
But the command on that post is not complete...

VBoxManage modifyvm vmname longmode off

So mine virtual machine is called "W7"...

VBoxManage modifyvm w7 longmode off

 In terminal...

"VBoxManage: error: Could not find a registered machine named 'w7'
VBoxManage: error: Details: code VBOX_E_OBJECT_NOT_FOUND (0x80bb0001), component VirtualBox, interface IVirtualBox, callee nsISupports
VBoxManage: error: Context: "FindMachine(Bstr(a->argv[0]).raw(), machine.asOutParam())" at line 455 of file VBoxManageModifyVM.cpp"

you can see this error is because of the lower case, again in terminal...

VBoxManage modifyvm W7 longmode off

"VBoxManage: error: Invalid parameter 'longmode'"

but if added -- at the beginning of longmode it will work and solve the problem.

VBoxManage modifyvm W7 --longmode off

I hope this help some one.