This assumes it’s installed and you have run the executable that adds it to your apps.
1. In AutoCAD execute command CUILOAD
2. Select MCOLOR9x and Unload it (eg. MCOLOR97 for M-Color 9.7)
3. Restart AutoCAD
If CUILOAD did not not restore M-Color menu:
1. Run AutoCAD commands: MCO_MCOLOR_MENU & MCO_MCOLOR_MENU2