It’s difficult to explain, really. I’ll let the picture do the talking for me:
As you can see, only a small portion of the model is visible. If I zoom out, even less can be seen, and the other way around if I zoom in.
You are in perspective view, and probably too close or inside the model. Switch to Orthographic view (Numpad 5 toggles between the two views) and see whether zooming works then.
Look in properties panel (N) for view clipping start and end distances. Ensure these are large enough to fit in your model. If your model is very large increase the clip end or scale down the model. When looking through the camera there are also start and end clipping distances in the camera properties
That did the trick. I can’t thank you enough.