I am not able to start the workflow from the campaign "workflow and targeting" tab, but if I go to Explorer view, under Administration>Production>Objects created automatically>Campaign workflow, I can select and run the workflow. Am I missing something here?
Can you be a little more specific about "I am not able to start the workflow"? Do you mean the execution buttons are grayed out? Or do you start the workflow and it doesn't run? Did you check if the Campaign Jobs workflow has failed as mentioned above? If so you can restart the Campaign jobs workflow.
I was referring to execution buttons which were greyed out. After restarting the Campaign jobs workflow, workflow started running.