Detecting this can be difficult if you have not selected in Options/FeatureManager to put the folder Solid Bodies on show mode (default is hide). This is because the folder for the solid body in the feature tree is not visible until there is more than one body in the part.
If the file that has disappeared in the graphic, is not a part or if the body has not been hidden, you may have experienced something more unusual, that the display information could not be generated correctly. First, verify that the model is not just zoomed out by running zoom to fit.
If the model does not appear now either, you can test an old support trick to trigger a calculation of the display information in the model by running normal to against one of your ground planes, followed by ctrl + Q. Usually the model will now be displayed.