Select a shape that already has a Sweep modifier using a Custom Section applied. > Modify Panel > Section Type rollout > Click Extract.
The Extract functionality allows you to recover custom cross-sections that may have been deleted from the scene. As long as you have a sweep in the scene that uses the deleted shape as a custom cross-section, Extract can be used to restore it to the scene.
In a large scene that has many objects and shapes, you can also use Extract if you want to quickly duplicate a section used by the sweep instead of searching for the original shape you used as the section.

Name—This field shows the default name that will be given to the extracted section. By default, it always has the naming convention of Sweep_ShapeName01, Sweep_ShapeName02, etc.
For example, if your missing section was named Roman-Ogee, the extracted shape will be named Sweep_Roman-Ogee01.
Copy—Places a copy of the extracted section at the global origin (0,0,0).
Instance—Places an instance of the extracted section at the global origin.
Reference—Places a reference of the extracted section at the global origin.