You can create a sectional view in two ways:
1. Define the cross section in the assembly model and then select the same while detailing.
2. Create a new cross section within the drawing environment using appropriate datum planes.
From your description of the problem, I understand that you do not have any pre-defined cross section in your assembly model. You can adopt the second method of creating the cross section. Feel free to post the query again in case of you are not clear about this topic.