Hello Stefan,
I am suspecting you have Photo Mechanic's Contact Sheet preferences set to Close the contact sheet window when the last tab is closed. This check box is in the Behavior section and should be unchecked to prevent Photo Mechanic from closing the contact sheet window. If you quit Photo Mechanic without any contact sheet window, then it will startup without opening a contact sheet. You can always use the Command-n shortcut or use the File menu's New Contact Sheet Tab, to open a contact sheet when none are present.
On Photo Mechanic's General preference page, go to the On startup section. If you set the option to Open empty contact sheet, then you will always see a contact sheet open when Photo Mechanic launches.
Best regards,
--Bob