If you have a blue or green USB dongle attached to your workstation, follow this flow:
If restarting the service fails, the drivers are likely corrupt. You need to run the official driver installer, which is still included with Mastercam 2021. mastercam 2021 license not found
Step-by-Step:
Mastercam 2021 sometimes conflicts with CodeMeter (used by other CAD/CAM software like SolidWorks or AutoCAD). If you have CodeMeter installed: If you have a blue or green USB