Having opened a file containing a Reality Mesh, all I see is in the View is a boundary box showing its location.
The issue in this instance is the path of the Reality Mesh (*.3sm) file had been changed, thus MicroStation was unable to locate to display the Reality Mesh. The same would be true if the file was renamed.
Move the Reality Mesh file back to its original location as shown in the Reality Mesh Attachments dialogs File Name field.
How to insert Reality Meshes into a 3D Design Model