Section cut issue

It seems I have a diagonal section in my drawing that appeared suddenly and I can’t find to remove it. When I zoom into an object to add details I loose a diagonal cut of the object & can’t seem to find out where this section cut is to remove it. Sketchup crashed and after the autosaved file is behaving this way. Is there something else causing this? Please help! I need to get this file done. Thanks! The file is too big to upload.

This is called Clipping, a search for clipping will find lots of threads.
There are many causes, some quite obscure.
One of the more common fixes is to change the Camera from Perspective to Parallel Projection, or the other way around.