I had some storm drain and sewer profiles. One day the structures disappeared, but the pipes didn't. This issue seems to have happened to other people as well, but nobody could find a solution. Until now.
I tracked down the pipe network catalog to a folder. You can locate this folder on your system by pulling down the Create Design tab on the Home panel of the ribbon and picking Set Pipe Network Catalog. You reach the same window through the main Pipes menu, which includes the command to Set Pipe Network Catalog. The path to the folder is in the Catalog Folder box. That same window lets you select the structure and pipe catalogs, which must be present in that folder.
Apparently, those catalogs have to be valid groups of files. I could select the Imperial Structures catalog, but I could only ever access the first batch. I guessed that the second one had become corrupted somehow, so the validation process just gave up. I never tried to edit my structure catalog, so I don't know what happened to it.
The solution was to get a copy of that entire folder containing that structure catalog and replace the original. After that, everything worked as before. If you don't have a co-worker with a good version, I suppose you can repair or re-install Civil 3D. There's no telling if or when the problem will recur, but at least now I have a fix.
Interesting side note: I was able to restore the structures temporarily by creating a block and redefining the profile structure style to use that instead of the part parameters. The block is a rectangle, size .25 horizontal by 1.0 vertical, with the insertion point defined where you set it in the style, for example, the rim is the center of the top side. Not only does defining this block display the structure, it makes the label visible too. The downside here is that changing the style obliges other users to deal with the same temporary fix instead of seeing the correct structure.