When do you see this error?

Goof! When you, or someone else, does not “properly” shut down your virtual machine!  It could also happen because the host system crashed.  However, I’m pretty sure you did it! 

“Could not open virtual machine: C:\…….\machinename.vmx. This virtual machine appears to be in use.”

Here’s what you are going to do; well, if you want to see the VM instance back up anyhow. 
1.  Navigate to the directory where the VM image configuration file in question exists.
            a) you will see four folders with .lck extensions.  The folders are created when you start the instance.  If the VM is properly shut down these folders will be automatically deleted, but if the VM was not shut down properly they will not be deleted and VM will think that the image is already started and not allow you to view/manage the instance.

2.  Okay, now that you are in the directory of said instance and you have located the offending .lck folders; delete them.

3.  Start your instance up and enjoy.

That wasn’t so hard now was it?