Cannot get my C & C to start.

Okay. Specifying which C&C might help?

Anyway, the most likely solution is: in Origin, open the game's properties, and disable the "Origin in game" feature.

This selection is greyed out for me to change. How can I get it to not be greyed out?

This means you tried to start the game, and Origin thinks it's still running.This is part of what goes wrong, really.

To fix that, either kill the launched game in task manager, or restart your PC and start Origin again. Then go and change the option..