You usually get this in cases the VM doesn't exit cleanly.
Exit Workstation and run "net start vmx86"
If that doesn't help - kill any orphaned vmware-vmx.exe processes you might have running and try the net start cmd again.

You can find out which vmware-vmx process belongs to which vmx-file by running sysinternals pipelist.exe

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description of vmx-parameters: http://sanbarrow.com/vmx.html
VMware-liveCD: http://sanbarrow.com/moa.html