KB000451: What does Error Message "MS is locked to another session" Indicate? - MS ChemStation 

Verify that all of the MS Chemstation sessions are closed, including Data Analysis. Use the Task Manager and select the Application tab to further verify that no sessions are present. If you feel uncomfortable in doing this, shut down Windows and restart. Do not start any Chemstation application with the exception of CAGBootP. (If your system uses BootP, CAGBootP Server must appear on the PC task bar for the next step to be successful.) First, try resetting the MSD by using the Menu buttons on the MSD display. Press the Menu Key until 'Controller menu' appears. Next, press the Item Key until ‘Reboot controller?’ is displayed. Finally, press the Yes/Select Key to reload the firmware. If the reboot is successful, and communication to the PC is established, you will see a series of messages on the MSD display. A final message of "Not ready" is normal until the ChemStation session is started on the PC. Please Note: In some cases, the controller cannot be rebooted in this manner. If this happens, power must be turned off using the MSD power button. Allow three to five seconds (no more) before restoring power.