Well we may need a bit more information than that, since that sounds like the blend file lost all the links. Did you move any folder on your computer? Rename any file? Rename any folder? The blend file follows the path on your folders and if something changes it looses its way and can't find anything.
Make sure you save the Blender file with the marked assets in the asset library location set in the file paths section of the preferences.
The Blender file you are using to build your city will be looking at this location for the assets. If the asset file is not there, then the assets can't be found and will not show up in the asset browser or the 3d view.
It sounds like your paths got messed up. Happens all the time when utilizing a linking workflow. If you saved your assembly .blend in a new location from where it was saved when you linked in your assets, I recommend saving it back in that original location and going to File > External Data > Make Paths Relative. Save the .blend and revert it to reload all paths