Missing model components

The model may be hidden in the viewport.
​Although this is a rare occurrence, artists sometimes hide the model in the viewport. To reveal the model, you would use the Unhide All function in your application. Some applications may offer several ways to hide objects. For example, in 3ds Max, you can hide an object through the quad menu, object properties, the layers menu, or hide by category.

The object may have transparency enabled.

If you're using an exchange format, be aware that imported files may appear with transparent materials. This is dependent on several factors, such as how the model was made, how it was exported, and what program it is being imported into. Disabling transparency value/channel may resolve the issue.

The model's origin point may not be set to 0,0,0.
​When you open a model, your program will default to the 0,0,0 point of origin in the viewport. If the artist did not set the model to this point of origin, the model may not initially display when you open it. This typically occurs with exchange formats. Using the Select All function along with Zoom to Selected/Zoom Extents may bring your viewport directly to the object(s).

Sections of the model may be hidden.
​In certain scenes with multiple or complex object setups, some sections may be hidden by default to help keep the workspace organized. These hidden objects can typically be revealed using the program’s unhide options.

Different programs may offer various ways to hide objects. For example, 3ds Max allows you to hide objects using the quad menu, object properties, the layers menu, or hide by category.